Synthesis of La_9.33Si₆O₂₆ nano-photocatalysts by ultrasonically accelerated method for comparing water treatment efficiency with changing conditions

1 Introduction

Albeit the progress of industry brings human supreme life, the environment is contaminated (Lee et al., 2016; Thakur et al., 2010). Water contamination has possessed excellent consideration of researchers, owing to the absence of fresh water sources in the universe. Now, people start to request novel ways of water treatment, and photo-catalysis is favorable and promising technique (Bhatkhande et al., 2002; Altaee and Alshamsi, 2020). The main components of solar energy are ultraviolet (λ = 200–400 nm), visible light (λ = 400–800 nm) and infrared (λ > 800 nm), and the proportion of which are approximately 5%, 43%, and 52% respectively. The ultraviolet energy can directly activate the chemical bonds of some organic molecules to give highly reactive radical intermediates, which results in poor selectivity of the products. Moreover, the infrared wavelength with comparatively low energy is unable to meet the requirement of activation energy for the vast majority of organic reactions (Chen et al., 2016). Visible light is abundant in nature compared to the ultraviolet and infrared, but it generally could not be adsorbed directly by reactant molecules to drive the reaction. Therefore, visible light photocatalyst employed as a bridging media for the energy transfer between visible light and substrates will be of particular importance (Djurišić et al., 2020). One principal mode of action in visible light photocatalysis is to induce an electron transfer to or from a substrate, thus generating radical anions or cations (or neutral radicals when cationic or anionic starting materials are used), which are often followed by the extrusion of a leaving group to overall form a neutral radical as the reactive species that initiates a chemical transformation (Marzo et al., 2018).

On the other hand, ecofriendly synthesis methods are noticeable subjects for prevent the pollution of surroundings (Alshamsi et al., 2021; Altaee et al., 2020). The sonication technique as an advantageous synthesis route exploited for procurement of components with different application and characteristics. Moreover, sonication technology used in fabrication materials with multi-function and attractive structures to modify the physicochemical virtues. Ultrasound principally applied as the facile procedure for altering structure, size, dispersing and functionalizing substances (Xu et al., 2013; Salavati-Niasari, 2005; Salavati-Niasari et al., 2009; Hosseinpour-Mashkani et al., 2012).

The ultrasonic power is usually expressed as the electrical input or output power to or from the ultrasound generator. The efficiency of the energy transformation depends not only on the equipment itself, but also on ultrasonication conditions. Therefore, the amount of acoustic energy delivered into the liquid medium cannot be measured solely by measuring the amount of electrical energy expended to produce the mechanical vibration. As all the ultrasonic energy transferred to a liquid is eventually converted into heat, the power transferred to the treatment medium can be measured calorimetrically. Calorimetry power output is measured by recording heat generated into the liquid by recording the increase in temperature with time. These methods are very reliable and have been used for this purpose by different authors (Jordens et al., 2015). Reported data indicate that the power output attained in the medium increases with the amplitude of ultrasonic waves and the hydrostatic pressure, but decreases with increasing temperature. The combined effect of these factors on the power is still unknown (Fuentes-García et al., 2021). The bubbles volume and life cycle are determined by ultrasonic power intensity in aqueous media. From its formation, growth, and final implosion as a result of their collapse, they induce hot spots production with internal local temperatures up to thousands of K and high-pressure differentials. At these conditions, the reactants have intensive interactions in extremely short time-lapse, cavitation events results in heating and cooling rates of more than 1010 K s⁻¹ and sonocrystallization can be improved. Secondary sonochemistry effects are dominant at 20–100 kHz ultrasound frequencies with a higher level of transient cavitation, whereas chemical effects are dominant in range of 200–500 kHz due to the generation of a large number of active bubbles. The effect of sonication parameters and obtained results in the previous literatures show a remarkable improvement of the obtained materials, positioning to the ultrasound-assisted method as an option for nanomaterials elaboration, modification, and optimization in a simple and economically (Raso et al., 1999).

Apatite-type have generalized formula of X_10−x(X'O₄)₆O_3−1.5x, where X is a rare-earth or alkaline metal, and X' is a p-block element including phosphor, silicon or germanium (Kim and Lee, 2012; Hosseini et al., 2013). The lanthanum silicates as apatite type material comprises of tetrahedral SiO₄, which create 2 distinguished channels parallel to the c-axis. insulated ions of oxide are situated in the greater channels, while smaller channels are settled by 9 concordant Lanthanum cations. La_9.33(SiO₄)₆O₂ (or formula of La_9.33Si₆O₂₆) in terms of stoichiometry has 6.7% open situation (cation vacancies) and 2 channel ions per unit cell. Vacancies bring positional constructional distortion and replace about 14% of the interstitial oxygens into the perimeter of the channels (Vincent et al., 2007; Vojisavljevic et al., 2014). As a subgroup of apatite materials, the series of silicate materials is known for its structural flexibility, the high melting point and the good chemical stability. Therefore, various of applications for apatite-type silicate materials have been explored. In the present literatures, synthesis methods, crystal structures and the optical properties including potential optical applications of silicate-based materials are studied (Blasse, 1966). Among the silicates, Lanthanum Silicate shows some delightful applications such as long-lasting phosphors, lasers, X-ray imaging, displays, environmental monitoring and is a successful candidate for coherent optical memory time domain (CTDOM) devices. However, reports in literature reveal that a minimum of work has been done on the rare earth silicate for its manifold of applications as wLEDs, display devices, catalyst, sensors and electrochemical performance of the above mentioned sample (Naveen Kumar et al., 2021; Kumar et al., 2021).

In the literatures, preparation of Apatite-type lanthanum silicates conducted through five methods of plasma synthesis (Gao et al., 2008), thermochemical (Kuo and Liang, 2012), hydrothermal (Noviyanti et al., 2015), mechanical (Lu et al., 2017) and sol–gel (Yan and Huang, 2007). These methods led to agglomerated structures with large size. According to the attractive structure of lanthanum silicates, we try to design novel nanosized structure for optimizing properties of La_9.33Si₆O₂₆ for applying in water treatment application. So, Amin-assisted sonochemical synthesis utilized for preparation of La_9.33Si₆O₂₆ nanoparticles by investigation of effect of sonication time and applying sonication method (non-stop or pulsed) on morphology and size distribution. Therefore, ideal obtained product used as photocatalyst degradation of different dyes. Impact of utilizable factors such as concentration of dye, catalyst dosage and pH studied on the dye degradation efficiency of sonochemically prepared La_9.33Si₆O₂₆ nanoparticles. The photocatalytic degradation mechanism inquired though study the scavengers for active agents. Therefore, ideal result obtained in the presence of 0.03 g La_9.33 Si₆ O₂₆ nano-photocatalysts in 10 ppm acid red 14 in the pH of 11 which proceeds in terms of mechanism with the help of superoxide species.

2 Experimental section

The lanthanum silicate nanoparticles prepared through a simple and rapid sonochemical procedure using tetraethylenepentamine (TEPA). TEPA as an alkaline agent with a large carbon chain can play the role of masking agent to prevent particle agglomeration. At the first step, the precursor of La(NO₃)₃·xH₂O and tetraethyl orthosilicate (TEOS) dissolved in the ethanol/H₂O and mixed under stirrer. The obtained transparent solution transferred under the ultrasonic probe with a power of 60 W (18KHz) for 10 min. During the using sonication waves, the TEPA was added to the reaction medium and the pH was adjusted at 10. Sonication time as an important parameter was studied on the lanthanum silicate products. The different time duration of 10, 15, 20 and 30 min selected for sonochemical synthesis studies. Also, the pulse effect of 0.1 s and 0.5 s studied on the structural properties on samples. Table 1 shows all applied parameters for sample preparation. The obtained precipitate centrifuged, washed and dried at 80 °C for 12 h. Finally, the acquired samples were heated in the furnace for 8 h and 800 °C. The photocatalytic tests were designed in the 50 ml dye solution with a specified concentration under UV light source (40 cm away) for 120 min. The specified dosage of catalysts dispersed in the dye solution, aerated and took place during 30 min. Sampling was performed at the specific time applying the spectrometer of UV–Vis for absorbance mensuration.

3 Structural details

3.1

3.1 Phase purity and chemical composition

In order to check the crystal phase cognition, X-ray diffraction analyses were accomplished on the acquired samples. Fig. 1(a-d) represents the pattern for XRD results of lanthanum silicate samples fabricated during 10, 15, 20 and 30 min sonication waves, respectively. All samples show related peaks of La_9.33 Si₆ O₂₆ that match with the reference code of JCPDS: 49–0443. However, the prepared samples at 10, 20 and 30 min show partial impurity of La₂O₃ (JCPDS: 73–2141). Also, the synthesized sample in 20 min shows 2 unknown peaks. As a result, the obtained sample at 15 min has a pure phase of La_9.33 Si₆ O₂₆ with a hexagonal crystal system and space group of P63/m. The spectrum of apatite type lanthanum silicate sample with the formula of La_9.33 Si₆ O₂₆ shows several bands in the range of 400–1000 cm⁻¹ which attributed to metal–oxygen bands (La—O and Si—O) (Noviyanti et al., 2015). The band at 3600 cm⁻¹ arises from the stretching modes of O—H bond displays the presence of hydrous species on the surface of the sample. Also, the bending vibration of O—H illustrates at 1632 cm⁻¹. Fig. 2, present FTIR spectrum of silicates compound resulted from ultrasonic-assisted synthesis using amine of TEPA. The presence of trace amine in the surface of nanoparticles show by detecting N—H and C—H at 3436 and 1482 cm⁻¹, respectively (Salavati-Niasari et al., 2009). The elemental composition of acquired lanthanum silicate ascertained by analysis of energy dispersive spectra. Fig. 3 shows the elements of La, Si and O which coincide with composition of sonochemically synthesized lanthanum silicate nanoparticles.

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Fig. 1

XRD pattern for La_9.33Si₆O₂₆ nanoparticles using diverse sonication time (a) 10 min, (b) 15 min, (c) 20 min and (d) 30 min.

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Fig. 2

FT-IR result for obtained La_9.33Si₆O₂₆ nanoparticles (sample 1).

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Fig. 3

EDS result for obtained La_9.33Si₆O₂₆ nanoparticles (sample 1).

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3.2

3.2 Formation and sonication mechanism

The function of ultrasonic dependent on acoustic cavitation includes bubbles formation, expansion and collapse of bubbles. Unstable temperature and pressure of bubbles can provide a suitable status for crystal growth in nanoscale. Therefore, ultrasonic waves cause nanosized structures with favorable surface properties. The occurrence of bubbles explosion led to shaking wave that dispenses into reaction surroundings. Also, a bubble outburst can rupture the spherical polarity of the bubble by creating of microjets that impressed on the external area. Suspension of Particles can speed up within the constructed vibration waves (illustrated in Scheme 1). In consequence, the capability of executing eminent changes in configuration, compound, and reactivity was arise by the interparticle encounters (Salavati-Niasari, 2005; Ghanbari and Salavati-Niasari, 2018; Zinatloo-Ajabshir et al., 2019).

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Scheme 1

Illustration mechanism for sonochemical synthesis of La_9.33Si₆O₂₆ nanoparticles.

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The formation for lanthanum silicate nanoparticles presented in the equation of 1–7 through step by step reaction mechanism. During the sonication process, in the first step homolytic cleavage of water takes place (Eq. (1)) and the H₂O₂ formed (Eqs. (2)–(4)) which reacted with TEOS in the second step (Eq. (5)) and led to the creation of ${\left(\mathit{Si}{O}_4\right)}^{4-}$ species (Eq. (6)) (Basavaraj et al., 2017; Zhang and Chang, 2010). In the third step the $\mathit{La}{\left(N{O}_3\right)}_3$ react with ${\left(\mathit{Si}{O}_4\right)}^{4-}$ in the ethanol medium and presence of TEPA that cause to precipitate the lanthanum silicate nanoparticles (Eqs. (6) and (7)).

(1)

H₂O → H° + OH°

(2)

H° + H° → H₂

(3)

H° + OH° → H₂O

(4)

OH° + OH° → H₂O₂

(5)

$$H_2{O}_2+Si{\left(\mathit{OEt}\right)}_4\to Si{\left(\mathit{OH}\right)}_4+4C_2{H}_{5}OH$$

(6)

$$\mathit{Si}{(OH)}_4+La{(N{O}_3)}_3.x{H}_{2}O+TEPA\to {\mathit{La}}^{3+}+{(Si{O}_4)}^{4-}+HN{O}_3$$

(7)

$${\mathit{La}}^{3+}+{(Si{O}_4)}^{4-}+TEPA\to {\mathit{La}}_{10-y}{(Si{O}_4)}_6{O}_{3-1.5y}$$

Furthermore, the action of TEPA will create hydrogen bonds with OH groups on the surface of La_9.33 Si₆ O₂₆. Hence, TEPA as a covering agent could easily adsorb on the La_9.33 Si₆ O₂₆ crystal facets and forbid independently growth of particles. Particle nucleus can grow on all directions. Utilizing masking agent surrounded by the nucleus controls the particle size caused by growth. In addition, the long carbon chain of TEPA keeps the particles apart from each other and forbids from agglomeration.

3.3

3.3 Effect of sonication time

Effect of sonication on the morphology and size of samples illustrate in Fig. 4 (a-d) for the ultrasonic duration of 10, 15, 20 and 30 min, respectively. The SEM images reveal that the synthesized sample in the 10 min is an ideal product. According to the drawn histogram for size distribution for sample 1, 2 and 3 in the Fig. 5(a-c), the average size for these samples is 68, 69 and 79 nm. Therefore, the ideal sample in terms of size is sample 1 which synthesized during 10 min sonication waves under 60 W power. As shown in Fig. 4d, the sample 4 (30 min sonication) has larger structures than other samples (1, 2 and 3) with undefined morphology. With enhancing the sonication time up to 30 min, the tiny particles agglomerated due to active surface with high energy and formed large structures. Therefore, decreasing sonication time led to prevention of particle growth and agglomeration which provided optimum condition for preparation of smaller nanostructures than other samples.

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Fig. 4

SEM images for La_9.33Si₆O₂₆ nanoparticles using diverse sonication time (a) 10 min, (b) 15 min, (c) 20 min and (d) 30 min.

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Fig. 5

Computed size distribution results for obtained for La_9.33Si₆O₂₆ nanoparticles synthesized in sonication time of (a) 10 min, (b) 15 min and (c) 20 min.

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3.4

3.4 Effect of sonication pulse

On the other hand, the effect of applying pulsed ultrasonic waves was investigated on the size and morphology of samples. Fig. 6 (a, b) shows the synthesized samples in the presence of 0.1 s and 0.5 s pulsed waves, respectively. By comparing the results of applying pulsed waved and without pulse effect, we conclude that samples which sonochemically synthesized through monotones waves have the small size in the uniform distribution. Transmission electron microscopy operated for support the structural properties of generated nanoparticles of La_9.33 Si₆ O₂₆ in monotones sonication time of 10 min. The TEM results in 150, 80, 40 and 20 magnifications (Fig. 7) picture the ideal crystal size in the average size of 40 nm.

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Fig. 6

SEM images for La_9.33Si₆O₂₆ nanoparticles using pulse time sonication during 10 min (a) 0.1 s and (b) 0.5 s.

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Fig. 7

Different magnification images for TEM results of La_9.33Si₆O₂₆ nanoparticles (sample 1).

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3.5

3.5 Surface characteristics

The surface specifications agreed with synthesized samples in 10 min sonication irradiation are depictured in Fig. 8. Fig. 5 plots the N₂ adsorption/desorption isotherms composing pore-size distribution of La_9.33 Si₆ O₂₆ nanoparticles. Conforming to IUPAC assortment, obtained isotherm was delineated in type IV. H3-type hysteresis was attached to the mesoporous structures that categorized for sample 1. The medium size of nanoparticles was appraised between 1 and 53 nm for samples 1 via employing the BJH scrutiny (insert in Fig. 8). The surface area for sample 1 is 9.32 m².g⁻¹ and the measured pore volume and pore size in average are 0.048 cm³.g⁻¹ and 20.95 nm, respectively.

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Fig. 8

N₂ adsorption/desorption isotherm and pore size distribution curve (insert figure) for La_9.33Si₆O₂₆ nanoparticles (sample 1).

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4 Optical and photocatalytic details

Band gap determination is important analysis for photocatalyst materials to recognition of type of light source in the photocatalytic process. The estimated band gap for samples 1 was studied through analogizing (K_Mhυ)ⁿ curve versus hυ to zero. The correlated curve is exhibited in Fig. 9. The appraised band gap value is around 3.25 eV for the synthesized La_9.33Si₆O₂₆ nanoparticles under non-stop sonication waves for 10 min. The band gap is important data for examination of the suitable type of lighting source necessitated for photocatalytic dye degradation. The selected light irradiation in the photocatalytic test series is ultraviolet due to calculated band gap of 3.25 eV. The photocatalytic experiments were designed by changing a series of conditions to comparing ideal efficiency and removing pollutants. Type of dye, dye concentration, catalyst dosage and, pH are the operational parameters studied in these experiments.

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Fig. 9

(a) UV–vis spectra and b) plot of (αhν)^1/2 versus (hν) for La_9.33Si₆O₂₆ nanoparticles (sample 1).

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4.1

4.1 Type of dye

The obtained photocatalytic data for effect of La_9.33Si₆O₂₆ nanoparticles on degradation of different dye spices are perceivable in Fig. 10 for Acid red 14 (Ar), Acid yellow (AY), Methyl orange (MO), Thymol blue (TB), Eriochrome Black T (ECBT) and Rhodamine B (Rh B). The destruction percent of mentioned dyes in the existence of La_9.33Si₆O₂₆ nanostructures as photocatalyst under source of ultraviolet light was calculated as 61.70% (Ar), 32.43% (AY), 39.52% (MO), 22.22% (TB), 53.70% (ECBT) and 4.03% (Rh.B). Therefore, the effect of La_9.33Si₆O₂₆ nano-photocatalyst on the degradation of Acid red 14 is higher than other dyes as an ideal organic pollutant model.

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Fig. 10

Photocatalyst study of La_9.33Si₆O₂₆ nano-photocatalysts for degradation of different organic dyes.

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4.2

4.2 Action of dye concentration and catalyst dosage

Effect of concentration of Acid red 14 and dosage of La_9.33Si₆O₂₆ nanocatalyst was examined on the photocatalytic activity of La_9.33Si₆O₂₆ nanoparticles which presented in Table 2. Fig. 11 (a-c) expresses the photocatalytic yield in the presence of 10, 15 and 20 ppm, respectively. The results show, the optimum condition for high degradation of Acid red 14 reached in the 10 ppm solution of Acid red 14 with dosage 0f 0.03 g of La_9.33Si₆O₂₆. By increasing the dye concentration, a greater number of dye molecules would be saturated the binding sites found on the surface of catalysts. It is obvious that the percentage of removal efficiencies decreased as the initial dye concentrations increased. In addition, an increase in catalyst loading can saturate the medium of dye solution and led to reaching irradiation prevention (Sunayana et al., 2010). This pheromone causes to decrees photocatalytic efficiency by loading 0.07 g catalysts. Finally, ideal amount for proper degradation yield is applying 0.03 g La_9.33Si₆O₂₆ nanoparticles.

Table 2 Summarized photocatalytic results for obtained La_9.33Si₆O₂₆ in different operational parameters.

Dye Concentration	Catalyst Dosage
	0.03	0.05	0.07
10 ppm	61.70%	36.52%	22.82%
15 ppm	21.78%	17.99%	26.35%
20 ppm	13.45%	23.76%	12.06%

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Fig. 11

Photocatalyst study in different dye concentration of acid red 14 using different dosage of La_9.33Si₆O₂₆ nano-photocatalysts (a) 10 ppm, (b) 15 ppm and (c) 20 ppm.

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4.3

4.3 Action of pH

Also, the impact of pH on the obtained ideal condition (10 ppm Acid red 14 with dosage 0f 0.03 g) was studied in Fig. 12. The solution of Acid red 14 with a pH of 11 was described better photocatalytic efficiency in the presence of 0.03 g La_9.33Si₆O₂₆ under UV irradiation. Alkaline medium (pH = 11) can be ascribed to enhance the negatively charged hydroxide ions. The negatively charged hydroxide ions lead to the development of a positive charge on the surface. In general, two incidents happen; 1) the most active sites on the catalyst surface produce and 2) hydroxyl radical concentrations enhance. The radicals of H₂O₂ and HO₂ were generated owing to the reaction of two radicals of OH and at high pH, the reaction between radicals of OH occurs in a simple way (Hussain et al., 2020; Ejhieh and Khorsandi, 2010). So, the obtained mechanism confirms that alkaline medium has a direct relation with the degradation of anionic dyes.

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Fig. 12

Photocatalyst study in different pH for La_9.33Si₆O₂₆ nano-photocatalysts in 10 ppm of acid red 14.

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4.4

4.4 Photocatalyst mechanism

During the photocatalytic process under an irradiation light, the generated pairs of e⁻ and h⁺ equip the acting operators for destruction of dye as bellows (Ansari et al., 2016):

(8)

$$\mathit{Catalyst}+h\nu \to Catalys{t}^{\ast }$$

(9)

O₂ + e⁻ → ⁰O₂⁻

(10)

⁰O₂⁻ + H⁺ → ⁰HO₂

(11)

2⁰HO₂ → O₂ + H₂O₂

(12)

H₂O₂ + e⁻ → ⁰OH + ⁰OH

(13)

h_VB⁺ + H₂O → ⁰OH + 2H⁺

(14)

Dye + ⁰OH → Degradation of dye

Three factors comprising radical of hydroxyl, holes and anions of superoxide can collaborate in the mechanisms and eliminate the contaminants (Zinatloo-Ajabshir et al., 2020). In order to study the process of photocatalyst for La_9.33Si₆O₂₆ nanoparticles, degradation test of Acid red 14 was performed in presence of benzoic acid (BA), Benzoquinone and EDTA as scavengers of OH⁰, ⁰O₂⁻ and h⁺ as acting operators, respectively (Zhang et al., 1998; Alshamsi and Alwan, 2015; Al-Bedairy and Alshamsi, 2018). Efficiency descent in existence of specifies of scavengers shows the task of type of acting operators in mechanism for photocatalyst. The percent of dye destruction in presence of EDTA, benzoic acid and Benzoquinone were ascertained in Fig. 13. According to obtained data, superoxide anion radicals was considered as the most active factor in this experiment. The percent of destruction reduced in the existence of the OH⁰ and h⁺ scavengers was partial.

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Fig. 13

Photo-stability of La_9.33Si₆O₂₆ nano-photocatalysts after 8 cycles.

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4.5

4.5 Photo-stability of catalysts

Stability and recycling of catalyst are essential issue for efficiency of photocatalyst materials. So, the cycle ability of La_9.33 Si₆ O₂₆ nanoparticles in 8 run of photocatalyst test was studied which repeated in the similar condition. The gained data express 49.76% for degradation of Acid red after 8 cycle (Fig. 14). It acknowledged that decreasing the absorption was induced by conducting the reactions related to e⁻-h⁺ pairs and optical procedures. The obtained data support the proper photostability of sonochemically synthesized La_9.33 Si₆ O₂₆ nanoparticles under UV light after 8 cycles.

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Fig. 14

Scavenger study for determination of photocatalytic mechanism through active agents.

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5 Conclusion

In summary, ideal La_9.33 Si₆ O₂₆ nanoparticles were synthesized by the sonochemical method in an optimum time of 10 min and power of 60 W using amine of TEPA. The average size for an optimum sample is 68 nm. The synthesized La_9.33Si₆O₂₆ nanoparticles used as catalysts for the degradation of organic pollutants in water. According to calculated band gap of 3.25 eV using DRS technique, the Ultraviolet irradiation used for degradation of dyes as pollutant models. The favorable efficiency (61.70%) was obtained in photocatalyst degradation of acid red 14 with a concentration of 10 ppm and presence of 0.03 g La_9.33Si₆O₂₆ nanoparticles adjusting pH = 11. Also, photocatalytic mechanism investigated in terms of determination of the type of active agent in remove organic pollutant. In this research, degradation of acid red was conducted through superoxide spices which produced during the process of photocatalyst.