Low-temperature synthesis, characterization and photocatalytic properties of lanthanum vanadate LaVO₄

Abstract

In this study, we have successfully prepared tetragonal lanthanum vanadate LaVO₄ nanoparticles by a facile co-precipitation method at room temperature. The obtained materials were characterized using different structural and micro-structural techniques such as the characterization by X-ray diffraction (XRD), UV–Vis diffuse reflectance spectrum (DRS), transmission electron microscopy (TEM), and Raman spectrometry. The obtained structure is crystallized in single tetragonal phase with pin-like nanostructure. A main optical transition with bandgap energy of 3.26 eV is evidenced, and the average lifetime of charges carriers was found to be 1 ns Furthermore, the photoluminescence occurs in the visible light range. The photocatalytic activity was evaluated by the photocatalytic degradation of methylene blue (MB) with initial concentration of 10 mg L⁻¹. The result indicates that LaVO₄ particles showed a best photocatalytic activity of 98.2% degradation for methylene blue solution after irradiation of 90 min under visible light. Furthermore, the photocatalytic mechanism and reusability were studied.

1. Introduction

Recently, water pollution has been a major issue for many societies across the World. However, The discharge of contaminants from various manufacturing sectors such as plastics, cosmetics, textile, as well as pharmaceutical and food industries [1,2] posed a serious threat to water resources [[3], [4]]. 800,000 tons of dyes are generated each year, and 140,000 tons are discharged in wastewater leading to adverse impacts on both human health as well as on the environment [5]. In this context it is reported that about 3.2 million persons die annually because of these poisonous contaminants [6]. Therefore, a number of physical, biological, and chemical water treatment procedures [7] have been employed such membrane filtration [8,9], adsorption [10,11], coagulation-flocculation [12,13], biological treatments [14] and ion exchange [15], in order to remove the organic dyes from water [16]. Unfortunately these conventional treatment processes are not effective for the mineralization of non-biodegradable substances and involve significant energy consumption and cost [17,18]. On the other hand, advanced oxidation technologies, especially photocatalytic degradation based on heterogeneous catalysis, have attracted significant interest because of its high efficiency regarding organic dyes, its easy and cost-effective setup. They are promising technologies for partial or complete mineralization of dyes into CO₂ and water [[19], [20], [21], [22], [23]]. Thus, photocatalysis is an efficient and sustainable technology to remove synthetic dyes from wastewater [16,24], operating under sunlight irradiation in order to create strong redox systems which are able to degrade the molecules in both aquatic and gaseous environments [25,26]. As is known, many researchers have developed photo-catalysts based on tungstates [[27], [28], [29], [30], [31], [32]], vanadates [33,34], molybdates [35], metal–organic frameworks [36,37] for the degradation of the organic pollutants and the depollution of wastewater [32]. Recently, ortho-vanadates (InVO₄, BiVO₄, CeVO₄, SmVO₄, FeVO₄ etc.) have received special consideration in photocatalytic degradation technology on account of the characteristics provided by the vanadium metal which exhibits electrons on the 3d orbital. These electrons can be activated by visible light and then participate in the decomposition reaction of organic contaminants [17,[38], [39], [40], [41]]. Among them, lanthanide orthovanadates LaVO₄ was considered as one of promising photocatalytic materials due to its catalytic performance, great optical properties owing to its unique electronic structure, and the absorption of visible light [[42], [43], [44]]. It primarily exists in two crystalline polymorphs, namely tetragonal phase (t) with zircon structure and monoclinic phase (m) with monazite structure [[45], [46], [47]]. LaVO₄ chooses the monazite type as a strong tendency because it is characterized by its higher oxygen coordination number of 9, in contrast zircon type with a number of 8 [45,48]. Thus, It is thermodynamically stable but it does not possess superior properties as compared to zircon [45,49]. The results show that t-LaVO₄ has an excellent thermal stability, a direct band gap, a small size of particles and high optical absorption. In contrast, m-LaVO₄ has an indirect band gap. As a result, it was confirmed that the promising properties of this structure have a positive influence on the catalytic properties.

Generally, LaVO₄ was prepared by sol-gel [44,50], hydrothermal [45], citrate method [51] and microwave methods [52] that used organic additives or surfactants and thermal treatment process [[53], [54], [55], [56]], which will increase the production costs and also produce new organic pollutants. The researchers developed a different number of catalysts by green synthesis such as (MgFe₂O₄@CoCr₂O₄ [7], CuO [2], MgFeCrO₄ [16], Mg_0.5Fe_0.5MnO₄ [24], Ni_0.25Fe_0.75Fe₂O₄ [57], NiFe₂O₄@ZnO [58] and CoMnCrO₄ [59]).

Herein, we report the preparation of non-calcined t-LaVO₄ at room temperature via co-precipitation process. The synthesis of LaVO₄ photocatalyst at ambient temperature is a novel synthesis strategy with a potential to address some of the challenges associated to traditional high-temperature synthesis methods. The as-synthesized catalyst was characterized by XRD, DRS, TEM, and Raman spectrometry. The photocatalytic activity of LaVO₄ was evaluated as a photocatalyst for Methylene Blue degradation in aqueous solution under light irradiation.

2. Experimental section

2.1. Materials and reagents

The reagents employed to synthesize these semiconductor photocatalysts comprise: lanthanum nitrate hexahydrate (La(NO₃)₃.6H₂O, 99.9%, Fisher Scientific), ammonium metavanadate (NH₄VO₃, 99.5%, ACROS Organics), nitric acid (HNO₃ 65%, Merck) and sodium hydroxide (NaOH, 99%, Fisher Scientific).

2.2. Synthesis of LaVO₄

The nanocrystalline LaVO₄ was prepared by a co-precipitation method. In a typical process, a quantity of NaOH and NH₄VO₃ were firstly dissolved in 20 mL distilled water to form a Na₃VO₄ aqueous solution. Then, La(NO₃)₃·6H₂O nitrate was dissolved in 15 mL of distilled water and 5 mL of 1 M nitric acid, this aqueous solution was added drop-wise to the first mixture under constant magnetic stirring. After 2 h of stirring, the pH value of the mixture was adjusted to about 7 by using sodium hydroxide solution. The mixture was maintained under continuous stirring for 30 min. The resultant solution was filtered as well as washed with distilled water and the precipitate was dried at 80 °C for 3 h and finely ground.

2.3. Material characterizations

The samples were characterized by X-ray diffraction (XRD; EMPYREAN Panalytical diffractometer) with Cu Kα radiation (λ = 1.5406 Å) ranging from 10 to 70° with a step size of 0.028° and a scanning speed of 0.001° 2ϴ.s⁻¹. Raman spectra were recorded at room temperature by using a RENISHAW spectrometer equipped with a 633 nm laser (30 s exposure time). Morphologies of the prepared samples were examined by transmission electronic microscopy (TEM: Tecnai G² 200 kV with a LaB₆ source). Electron dispersion spectroscopy (EDS) connected to the SEM was employed to observe the chemical composition of the samples. UV–vis diffuse reflectance spectra (DRS) obtained in the wavelength region of 200–800 nm, using UV–Vis spectro-photometer (JASCO UV–Vis V-730). The luminescence and the time-resolved fluorescence emission decay measurements were collected through fluorescence spectrophotometer (FLUOROMAX, HORIBA).

2.4. Photocatalytic procedure

The photocatalytic activity of as-fabricated LaVO₄ sample was evaluated by the degradation of methylene blue (MB) under light irradiation using Philips lamps 300 W. Firstly, 100 mg of LaVO₄ was dissolved in 100 mL of MB solution (3.1 × 10⁻⁵ M) in pH of 4.85. Then, the solution was sonicated for 15 min and agitated in dark for 1 h to get absorption/desorption equilibrium of MB with LaVO₄. Then, the reaction mixture was illuminated by visible light Philips lamps (300 W) under constant stirring using a magnetic stirrer. 3 mL of the suspension was harvested and centrifuged for 12 min at 13 200 rpm to separate the LaVO₄ powder from the solution. The decomposition evolution of MB was followed with the Shimadzu UV 2600 spectrophotometer registering the absorbance in the wavelength range 200–800 nm. In this (l-ascorbic acid; O2⁻), isopropanol (IPA; OH^.), disodium ethylenediaminetetraacetate (EDTA; h⁺) were used as reaction scavengers to indicate the contribution of active species into the photocatalytic pathway.

2.5. Zero charge point measurement

The point of zero charge pH_pzc corresponds to the pH value at which the surface charge is equivalent to zero. 50 mg of the sample was placed in six flasks that contained 50 mL of a 0.1 M potassium nitrate solution. The initial pH value of these solutions has been tuned to 2, 4.07, 6.27, 8.27, 10.13 and 12.09 using a few drops of 0.1 M nitric acid (HNO₃) or 0.1 M sodium hydroxide (NaOH). The resulting solution was stirred for 24 h. The resulting suspension was filtered, and then the final pH value was measured.

2.6. Chemical oxygen demand (COD)

MB photo-decomposition tests have been carried in a 100 mL beaker at ambient temperature. The bath experiment involves the solution to be treated, at a concentration of 10 mg/L, with amount of photo-catalyst. The chemical oxygen demand (COD) was read using a Lovibond COD kit and the resulting data was interpreted using an MD 200 COD spectrophotometer. The photo degradation rate was given as COD_t/COD₀ versus time. This rate was calculated for several reaction times by the formula in equation (1) below:

$$COD(\%)=\left(\frac{{COD}_0-{COD}_t}{{COD}_0}\right)*100$$ (1)

COD₀: COD concentration (mg/L of O₂) before degradation and COD_t: COD concentration (mg/L of O₂) at a value of time t.

3. Results and discussion

3.1. Structural analysis

Fig. 1 presents the XRD pattern of the obtained sample, all the peaks could be indexed according to the tetragonal phase of LaVO₄ (JCPDS No. 411083). It displays peaks at 18.01°, 23.93°, 30.19°, 32.23°, 34.06°, 38.82°, 43.18°, and 47.59° that correspond to the (101), (200), (211), (112), (220), (301), (103), (312) crystal planes of tetragonal LaVO₄, respectively. These results are in good consistency with those of Jie et al. [60].

Fig. 1.

XRD pattern of t-LaVO₄ synthesized by co-precipitation.

We calculated the size of the crystallites using the Scherrer method, which can be expressed as in equation (2) [22,61]:

D = kλ/βcos(θ) (2)

In this equation, D represents the size of the crystallite, λ is the wavelength of radiation and θ is the Bragg angles in radians. The integral breadth β value was calculated using a Gaussian approximation of peak profiles such as ${\beta }^2{=\Delta \left(2\theta \right)}_{samp}^2-{\Delta \left(2\theta \right)}_{stand}^2$. ${\Delta \left(2\theta \right)}_{samp}^2$ and ${\Delta \left(2\theta \right)}_{stand}^2$ are respectively the full width at half maximum (FWHM) of Bragg peak of the synthesized samples and the standard sample.

The constant k has a value of 0.9 and assumes that the profiles are gaussian-like. The calculated crystallite size was 18 nm.

3.2. Vibrational analysis

Raman spectra reported that peaks at about 200 cm⁻¹ and 1000 cm⁻¹ Fig. 2, as a result, seven Raman modes have been detected, they could be attributed to t-LaVO₄ phase according to reported data [62,63]. The peaks observed at low values (under ∼500 cm⁻¹) can be assigned to La–O vibrations, the high signal at ∼ 866 cm⁻¹ corresponds to the stretching mode of the V–O (ν1(Ag)) bond. On the other hand, the theoretical group computations 33 optical modes which contain 12 Raman active modes: 5 Eg + 4B_1g + 2A_1g + B_2g. These modes may be divided in internal (ν₁, ν₂, ν₃ and ν₃) and external (translational T & rotational R) modes of the VO₄ units as following [63]:

Fig. 2.

Raman spectrum of t-LaVO₄ NPs obtained at room temperature.

U = A_1g (v₁, v₂) + B_1g (2T, v₃, v₄) + B_2g (v₂) + E_g (2T, R, v₃, v₄). We conclude that the observed Raman bands correspond to those of the t-LaVO₄ phase which confirms the results of XRD analysis.

3.3. Morphological analysis

The morphology of t-LaVO₄ has been investigated by TEM observations (Fig. 3a). It is difficult to determine exactly the form of the particles: some of them are elongated (large:10 nm, length: 25–30 nm) and the other are shapeless. Moreover, they are piled on top of each other as can be seen in Fig. 3a. The size of individual particles was not measured. The electronic diffraction pattern Fig. 3b corresponds to a diffraction of several particles, it shows a series of punctuated rings, this look is characteristic of small crystallized grains. All the diffracted spots correspond to the planes of the tetragonal phase of LaVO₄ (they are labelled on Fig. 3b). These results are in good agreement with those of the X-Ray diffraction. We can deduce the crystalline character of the synthesized t-LaVO₄. The chemical composition of the material was control by EDS analysis (Fig. 3c). The semi quantitative analysis results are: 59 ± 1 at.% of La and 41 ± 1 at.% of V which leads to an excess lanthanum composition. This difference with the nominal composition may be due to an overlap of the lanthanum and vanadium peaks in the EDS spectra and therefore to an erroneous deconvolution. To refine the composition, it will be necessary to make an analysis with a device better resolved in energy.

Fig. 3.

(a) TEM image (b) electron diffraction pattern and (c) EDS spectra of t-LaVO₄.

3.4. Optical characteristics

Fig. 4a illustrate the diffuse reflectance spectrum of LVO with an outstanding optical absorption in range from 267 nm to 552 nm. This excellent absorption was due to the charge transfer transitions (CT) involve electronic transition from O to V ligands inside the VO₄³⁻ groups [64]. Generally, in vanadate semiconductors, the conduction band is mostly completed by V-3d electrons, whereas the valence band is mostly completed by O-2p electrons. The band gap energy (Eg) has been calculated by using the Tauc’s equation (αhν)^1/γ = B (hν − Eg) [65], in which (h: Planck’s constant, α: absorption coefficient, ν: frequency of the photon, and γ: coefficient = 1/2 or 2 for both direct or indirect band gaps). Several studies indicated that tetragonal LaVO₄ has a direct gap band gap (γ = 1/2) [66]. The value E_g = 3.26 eV was estimated from the plot of (αhν)² as function of hν (Fig. 4b).

Fig. 4.

(a, b) DRS and Tauc’s spectra of t-LaVO₄ nanoparticles. (c) PL graph of t-LaVO₄. (d) TRPL (lifetime-decay) curve of the LaVO₄ sample prepared at room temperature.

Photoluminescence spectroscopy (PL) is linked to the activity of photo-generated charge carriers. As can be noticed in Fig. 4c, The luminescence spectra show two main emission bands centered at respectively at 467 nm and at 568 nm. According to the UV–Vis spectra, the low emission at 467 nm corresponds to the band-edge electronic transition around the bandgap, while the strong emission at 568 nm is due to the transition from the defects energy levels localized in the bandgap.

. These emissions are related to the charge transfer from oxygen 2p to vanadate 3d orbitals inside the tetrahedral VO₄³⁻ groups [67]. They might also come from defects generated by other V ions [68]. The decay curve for LaVO₄ is presented in Fig. 4d. We should note that the excitation wavelength was set at 284 nm while the emission wavelength was set at 468 nm. The fitting of the decay plot using two models of mono-exponential and bi-exponential decay equations showed that the decay analysis follows a bi-exponential decay. The bi-exponential fit was performed according to equation (3) that is given as follows:

I_t = I₁ e^−(t/τ1) + I₂ e^−(t/τ2) (3)

wherein I₁ and I₂ present the intensities on different times, τ₁ and τ₂ their respective lifetimes. The mean lifetime has been computed by equation (4) [69].

$${\tau }_{moy}=\frac{\left(I_1{\tau }_1^2+I_2{\tau }_2^2\right)}{\left(I_1{\tau }_1+I_2{\tau }_2\right)}$$ (4)

The average lifetime of the prepared LaVO₄ nanoparticles was 1.108 ns?

4. Photocatalytic activity of LaVO₄

4.1. Photolysis and adsorption study

To demonstrate the performance of our LaVO₄, it is necessary to evaluate the direct adsorption of the contaminant in the dark. The photolysis study allows the identification of the photocatalytic decomposition in our operational conditions. In this sense, we have done a preliminary study to verify the part of adsorption and photolysis of pollutants (MB). Fig. 5a displays the absorption spectra of MB in the presence of LaVO₄. The adsorption process of MB on LVO surface does not exceed 6% after 3 h of contact indicating a very low adsorption of MB on LaVO₄ photocatalyst. The direct photolysis experiment was performed by MB solution with a C₁ of 10 mg.L⁻¹ under UV–Vis irradiation. Fig. 5b shows the evolution of MB spectra after 2 h irradiation and only a 4% decomposition of MB is achieved. This confirms the photodegradation in presence of LaVO₄ was better than direct photolysis and adsorption process.

Fig. 5.

(a) Absorption spectrum of MB in the presence of LaVO₄ catalyst and in the absence of UV illumination. (b) photocatalytic decomposition of MB under UV illumination in the absence of LaVO₄ particles.

4.2. Photocatalytic efficiency and mechanism of t-LaVO₄ nanoparticles

Herein, the photocatalytic activity of LaVO₄ particles was determined by the photo-degradability considering illumination time of an aqueous MB solution (10 mg.L⁻¹). Fig. 6a displays the visible absorption spectra of methylene bleu solution for several time in the presence of t-LaVO₄ NPs. Before any irradiation, the solution should be magnetically agitated for 1 h in darkness to set up the adsorption-desorption equilibrium. The photo-degradation procedure was investigated by measuring the intensity of the absorption bands (664 nm for MB). The intensity of absorption band depends on concentration of selected pollutant. The photo-degradation efficiency was identified by the ratios C_t/C₀ (eq. (4)), in which C_t and C₀ presented the concentrations of LaVO₄ at times t and t = 0. The type of kinetics has been evaluated by the relationship in equation (5):

$$\ln \frac{C_t}{C_0}={-\mathrm{k}}_{\mathrm{o}\mathrm{b}\mathrm{s}}\mathrm{t}$$ (5)

In the present relation, k is first order kinetic constant (Langmuir-Hinshelwood model). As can be observed in (Fig. 6a), presented the UV–Vis absorption spectra. It was observed that the absorption peak gradually decreased with increasing irradiation time. They indicated that the reduction in the concentrations of our C_t organic dye is due directly to photocatalytic decomposition. After 90 min of illumination the photocatalytic efficiency obtained by the C_t/C₀ ratio reached 98% (Fig. 6b), this performance is the result of both photolysis and photo-catalysis effects. Fig. 6c illustrates a linear correlation between ln (C₀/C_t) and t which can be easily seen with a rate constant of 0.043 min⁻¹.

Fig. 6.

(a) UV–Vis absorption spectrum of a solution comprising 100 mg of LaVO₄ photocatalyst and 10 ppm of pollutant. (b) Variation of the C_t/C₀ report versus time irradiation for the pollutant MB. (c) pseudo-first-order kinetics of the photodecomposition mechanism. (d) Measurement of the zero-point charge of LOV particles: pH_pzc = 6.49.

To explain photocatalytic degradation efficiency of MB, we have determined the pH_pzc (pH at the zero charge point) Fig. 6d The zero charge point depends on acid-base characteristic of surface material [70]. In our case the tetragonal LaVO₄ has PZC at pH 6.49 and MB is a cationic dye. For pHi values under or upon pH_pzc the LaVO₄ charge surface is positive or negative respectively. This means that the surfaces of LaVO₄ can be positively or negatively charged based on the pH of the surrounding environment. In our case the pH of our solution is around 9 which is higher than pH_pzc, thus, LaVO₄ becomes negatively charged. As a consequence, a high removal efficiency, because of the electrostatic attraction forces occurred between the catalyst surface negatively charged and cationic dye. Moreover, under basic conditions, the efficiency of photo-degradation is more important due to the presence of hydroxyl ions which are necessary for hydroxyl radical’s creation. Besides that, this high efficiency is attributed to the specific morphology, which is characterized by small and well crystallized grains of tetragonal phase. It is interesting to mention that the as-prepared photo-catalyst without calcination exhibit good photocatalytic performance by providing small particle size, high specific surface area and more surface sites available for charge transfer [71,72]; these results are in good accordance with the SEM/TEM and XRD results. On the other hand, the optical characteristics of LaVO₄, notably its excellent visible light absorption, led us to use a lamp similar to solar irradiation. Furthermore, the photoluminescence study revealed strong emissions in a wide range of colors from blue to red, whose composition was influenced by surface defects, such as oxygen vacancies and variations in the La³⁺ surroundings.

Table 1 gives the photocatalytic efficacy of LaVO₄ based catalysts reported in the literature employing different synthesis methods, examined contaminants and light sources. The table shows that the photo-catalyst developed in this study has a similar photocatalytic activity as the other LaVO₄-based catalysts discussed in the literature. This activity was explained by the effective separation of the electron-hole (e⁻; h⁺) pairs under irradiation.

Table 1.

Photocatalytic activity comparison of the LaVO₄ photocatalyst with some documented materials.

Catalyst	contaminant examined	Synthesis Method	Operating Conditions (C0; Light Source)	Degradation Efficiency; Time	Ref
m-LaVO4	MB	Solventless	10−5 M, ---	82.2%, 90 min	[73]
m-LaVO4	MB	hydrothermal	1.5 10−5 M, 400 W metal halide lamp (λ = 510 nm)	91%, 60min	[74]
m-LaVO4/BiOBr	RhB	hydrothermal	3.1 10−5 M, 500 W Xe lamp	83.37%, 60min	[75]
t-LaVO4/g-C3N4	RhB	hydrothermal	3.1 10−5 M, 350 W Xe lamp	99%, 90min	[66]
CNTs/LaVO4	sulfamethazine	hydrothermal	10 10−5 M, 400 W metal halide lamp	70%, 90min	[76]
LaVO4/TiO2	Benzene	Sol-gel	3.2 M, 500 W Xe-arc lamp	57%, 600min	[77]
t-LaVO4	MB	Ambient Synthesis	3.1 10−5 M, Philips lamps (300 W)	98%, 90min	This study

The following empirical equations (6), (7) were used to calculate the positions of the valence band (VB) and conduction band (CB) edges of a semiconductor [78]:

E_VB = X − E^e + 0.5E_g (6)

E_CB = E_VB − E_g (7)

where, E_CB is the conduction band edge, E_VB is the valence band edge, X is the electronegativity (5.74 eV) [79], E^e is the energy of the free electrons on the hydrogen scale (approximately 4.5 eV), and E_g is the semiconductor band gap energy (3.26 eV). The calculated E_CB and E_VB values are (−0.39 and 2.87 eV) For the LaVO₄ catalyst.

The calculated valence and conduction bands provided valuable insights into the proposed photocatalytic mechanism, also for the role of superoxide anion radicals in the photo-activity of LaVO₄. According to literature, the valence band of LaVO₄ is mainly composed of the O2p orbitals, while the conduction band is mainly composed of the V 3d orbitals [80]. Herein, the calculated bandgap of LaVO₄ is around 3.26 eV, as shown in Fig. 4b. Based on this information, it is proposed that LaVO₄ absorbs visible light and generates electron-hole pairs in the conduction and valence bands, respectively. The photo-generated electrons and holes can react with adsorbed species, such as water and oxygen, to generate superoxide anion radicals and hydroxyl radicals. The calculated valence and conduction bands show that superoxide anion radicals play a major role in the photo-activity of LaVO₄.

4.3. Effect of catalyst dosage and pH

The effects of catalyst dosage and pH on the degradation efficiency were studied. As it is known, as property of heterogeneous photo-catalysis, the catalyst dosage influenced the decolorization of wastewater. The photodecomposition of the pollutant increased with the quantity of catalyst [81]. Increasing the amount of photocatalyst effectively causes an excess of active sites on the catalyst surface, leading to creation of –OH radicals that may contribute to the actual discoloration of the pollutant solution. Above a certain threshold of catalyst, the solution turns cloudy, thereby blocking the UV radiation for the reaction to take place, and the rate of degradation begins to decrease [82]. Fig. 7a illustrates the photo-degradation of MB dye at various amounts of LaVO₄ photocatalyst. As can be clearly observed, the rate of photo-degradation rises as the quantity of photocatalyst rises. The less amount of photocatalyst (50, 75 mg), the lower the active species formed and therefore a lower rate of degradation was observed, 33 and 73% for 25 and 75 mg respectively during 90 min of irradiation. When the quantity of catalyst is raised to a certain value (150 mg), the capacity of the catalyst to receive photons is saturated and the number of electrons and holes no further increases, so there is no improvement in the degradation rate. When the catalyst amount is great enough, it has a screening and diffusing action on the incident light. It should be noted that the optimum mass for degradation of a 10 ppm concentration of MB is 100 and 125 mg, but from an economic point of view, it is preferable to use a mass of 100 mg. The kinetic study (Fig. 7b and c), showed that the apparent constant increased with the amount of photocatalyst but it started to decrease at 150 mg. The reaction constants were 0.0043, 0.0146, 0.043, 0.0536 and 0,0119 min⁻¹ for 50, 75, 100, 125 and 150 mg, respectively. In addition to the effect of the amount of photocatalyst, the pH of the solution seems to play a major role in the photocatalytic process. The effect of pH on the photodecomposition of MB was examined in the pH range 3–9. An essential factor in the photocatalytic processes that take place on the surfaces of particles is the pH of the solution, as it dictates the surface charge characteristics of the photocatalyst [83]. Therefore, pH has an important influence on both the characteristics of dyes and the reaction mechanisms which may participate in dye decomposition, i.e. hydroxyl radical attack, reduction via the electron in CB and direct oxidation by the holes. Fig. 7d shows that the photocatalytic degradation of Methylene Blue increases with increasing pH, confirming the results obtained from the PZC measurement of LaVO₄ (Fig. 6d). On the other hand, the increase in pH leads to an increase in OH° radicals, which is beneficial for the photocatalytic process. The photo-degradation of MB is maximal at pH = 9 because it is a cationic dye and the surface of the photocatalyst at this pH point is negative, hence a strong interaction between the negatively charged photocatalyst and the MB of cationic nature. The kinetic study (Fig. 7e and f) showed that the apparent constant increased with the amount of photocatalyst but it started to decrease at 150 mg. The reaction constants were 0.0064, 0.0099, 0.0303 and 0.043 min⁻¹ for 3, 5, 7 and 9, respectively.

Fig. 7.

Photocatalytic degradation of MB in the presence of various quantity of LaVO₄ photocatalyst (a, b, c) and different pH of solution (d, e, f).

4.4. Trapping test and stability

Radical capture experiments were studied. Isopropanol alcohol (IPA), l-ascorbic acid, ethylene-diamine-tetra-acetic acid disodium (EDTA 2Na) were adopted as h⁺, superoxide radical (·O2⁻) and hydroxyl radical (*OH) scavenger, respectively. Fig. 8a illustrates the effect of the scavengers on the degradation process. The photocatalytic decomposition efficiencies of MB were 98% in the absence of scavengers, while in the presence of IPA, EDTA and l-ascorbic acid, the rate value reduced to 70%, 35.4% and 14.1%, respectively. Therefore, it can be concluded that holes (h⁺) and superoxide (O₂^.-) are essential for the photodecomposition of methylene bleu and hydroxyl radicals (OH^.) has not greater influence. Similar results confirmed by Samy et al. [74].

Fig. 8.

(a) Photocatalytic degradation of MB by LaVO₄ in presence of trapping agents. MB = 10 mg.L⁻¹, illumination time = 90 min, scavenger = 4.10⁻³ mol.L⁻¹. (b) A schematic diagram illustrating the suggested decomposition mechanism for LaVO₄ catalyst. (c) photo-catalyst recycling test. (d) XRD analysis of LaVO₄ photo-catalyst after photocatalytic application.

According to these obtained results, the proposed mechanism of photocatalytic decomposition is shown in Fig. 8b. The holes (h⁺) and electrons (e⁻) produced under visible light irradiation react with the molecules presented in the solution such as H₂O, O₂ and OH⁻ to generate highly active OH^. and O₂^.- ions respectively. The results obtained by Jilani et al. [84]., confirmed that reactive species break down the MB molecules into smaller entities, that lead to complete mineralization (CO₂+ H₂O). The photogenerated electrons of LVO have a higher reduction potential than O₂^•−/O₂ (−0.28 eV). As a result, when oxygen molecules (O₂) are adsorbed on the surface of LVO, the released electrons can easily transfer to them, generating O₂^•− superoxides. These latter are strong oxidizing species that can degrade organic molecules. Furthermore, it is also possible to produce OH^• hydroxyl radicals directly from photogenerated holes. This is because the valence band position of LVO (2.87 eV) is more positive than that of H₂O/OH^• (1.99 eV) [85]. Therefore, the holes on the LVO surface can also react with OH⁻/H₂O to form free radicals in the photocatalytic reaction. However, O₂^•− superoxides have a stronger reduction capacity and can react with H₂O or OH⁻ to produce OH^•. This is in agreement with scavengers trapping measurement which shows the predominance of O₂^•-efficiency followed by the holes.

The stability of LaVO₄ was examined by recycling the catalyst for MB decomposition under visible light irradiation. After each reaction, the recovered photo-catalyst was cleaned with deionized water and then was dried for reuse. Fig. 8c presented the test results, which showed a reduction in photo-activity after four successive experiments. The little decrease is probably due to the loss of photo-catalyst during recycling or to the molecules of organic dye MB adsorbed on catalyst surface, as a result of blockage of active sites in process of capturing [86]. XRD analysis Fig. 8d verified that the crystal structure has not altered after the photocatalytic processes.

4.5. MB mineralization via COD analysis

The identification of the Chemical Oxygen Demand (COD) used to determine the level of mineralization of the contaminants after the photocatalytic oxidation. It provides a complementary view of the photocatalytic performance of the photo-catalysts. Fig. 9 illustrates the removal rates of COD for LaVO₄ photo-catalyst used in photo-degradation of MB. In terms of COD elimination (Fig. 9a and b), COD removal was 85% after 1 h 30 min of UV–visible illumination for 100 mg of LaVO₄, 10 mg/L initial MB concentration at pH 9 and 90 min irradiation time.

Fig. 9.

a, b) Analysis of chemical oxygen demand removal versus time using LaVO₄ photo-catalyst, pH = 9, photo-catalyst mass = 100 mg, [MB] = 10 mg/L, irradiation time 90 min.

This trend is in agreement with the scavenger tests that justify the major role of the O₂⁻ radicals. So obtaining this COD removal (85%) rate indicates that this photo-catalyst is able to successfully convert MB molecules into CO₂ and H₂O [66].

5. Conclusion

As a summary, lanthanum orthovanadate was prepared successfully by facile co-precipitation method at room temperature. The synthesized LaVO₄ crystallizes in tetragonal structure with pin-like nanostructure with a size lower than 14–18 nm. The optical characterization shows a direct gap energy 3.26 eV and an average life time of 1.108 ns, also the particles have interesting photoluminescence characteristics when they are irradiated by UV light.

The photocatalytic activity analyses indicated that the LaVO₄ prepared to have good photocatalytic activity, degradation of the methylene blue solution (99%) of concentration 10 mg.L⁻¹ during 90 min of irradiation. The enhanced activity could be attributed to the nanometric size of the particles which offer more active sites as well as its capacities to absorb light in the visible range. Radical trapping investigations showed that superoxide and holes species contributed to the decomposition of MB. The reusability of LaVO₄ samples was evaluated in four consecutive runs indicating great stability of the photocatalyst after long photocatalytic reaction periods.

Author contribution statement

Safia Lotfi, Mohamed El ouardi: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Hassan Ait Ahsaine: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Veronique Madigou: Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Amal BaQais: Analysis tools or data; Interpreted the data; Wrote the paper.

Abderrazzak Assani, Mohamed Saadi: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper.

Madjid Arab: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper.

Data availability statement

Data will be made available on request.

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