Tailoring of Ag<sub>3</sub>PO<sub>4</sub>-Anchored Hydroxyapatite Nanophotocatalyst with Tunable Particle Size by a Facile Ion-Exchange Method for Organic Textile Dyes Photodegradation

Dhatchayani, Surendran; Sankaranarayanan, Krishnasamy; Kathiresan, K.

doi:https://doi.org/10.1155/2023/9882964

Journal of Nanomaterials

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Abstract Introduction Experimental Results and Discussion Conclusion Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

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Eco-friendly Waste-based Nanocatalyst Materials in Energy, Environment and Biological Applications

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Research Article | Open Access

Volume 2023 | Article ID 9882964 | https://doi.org/10.1155/2023/9882964

Tailoring of Ag₃PO₄-Anchored Hydroxyapatite Nanophotocatalyst with Tunable Particle Size by a Facile Ion-Exchange Method for Organic Textile Dyes Photodegradation

Surendran Dhatchayani,¹Krishnasamy Sankaranarayanan,¹and K. Kathiresan²

Academic Editor: Balasubramani Ravindran

Received05 Aug 2022

Revised26 Sept 2022

Accepted06 Oct 2022

Published10 Apr 2023

Abstract

Silver phosphate (Ag₃PO₄) exhibits excellent photocatalytic performance but has limitation in its stability and reusability. To overcome the issue of reusability, composites of silver phosphate nanostructures are tailored. This paper elucidates the photocatalytic study of silver phosphate-anchored hydroxyapatite (HA) (Ca₁₀(PO₄)₆(OH)₂) on the degradation of commercial textile dye, Sunfix Red (SR) S3B 150% by changing the experimental parameters such as catalyst concentration, dye concentration, and pH of the dye solution under the sunlight. Silver phosphate-anchored HA (HA_Ag₃PO₄(x)) was prepared via a two-step process. HA was synthesized by a simple wet chemical precipitation and silver phosphate-anchored HA (HA_Ag₃PO₄(x)) via in situ ion-exchange method. The synthesized photocatalysts were subjected to characterization studies such as X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, transmission electron microscopy (TEM), ultraviolet–visible spectroscopy, and X-ray photoelectron spectroscopy (XPS). All the synthesized composites exhibited the bandgap of 2.34 eV and degraded SR in 45 min with the rate constant 0.07168 min⁻¹ under sunlight. The trapping tests for radicals were done to study the role of free radicals in the degradation of the dye, SR, and the possible degradation mechanism was proposed. The postphotocatalytic analysis of XRD showed that the structure of Ag₃PO₄ remained intact declaring its structural stability. It was observed that the concentration of AgNO₃ precursor influenced the number of nucleation over the surface of HA and the particle size of Ag₃PO₄. The applicability of the synthesized material was extended to other organic dyes such as Sunzol Black (SB) DN conc., methylene blue (MB), and rhodamine B (RhB) by the prepared composite and the findings were presented.

1. Introduction

Exploitation of all kinds of natural resources and their replenishment is the major considerations in this age. Among these issues, water pollution and scarcity are making the situation hectic since water is an essential element of social and economic development. Therefore, many efforts have been devoted in order to conserve and sustain water resources. Urbanization and industrialization are not only exploiting the water resources, but also releasing effluents into water to such an extent that it is creating a great threat to the flora and fauna as well as humankind. The water contaminants include a wide range of chemicals, pathogens, and physical factors such as temperature, etc. [1, 2]. Textile industry is an important water-contaminant source as it consumes huge quantity of water and releases massive effluents. For the past decades, the researchers are paying much attention to the removal of textile organic pollutants by various methods which include biodegradation, electrocoagulation, photodegradation, oxidation, adsorption, and ultrafiltration [3–8]. Among them, photocatalysis has proved to be an excellent method to decompose the organic effluents being highly efficient and cost-effective [9, 10]. Due to photocatalytic efficiency and cost-effectiveness, TiO₂-based photocatalysts have been of research interest yet their practical application is limited because of their higher bandgap energy which falls in the UV region [10]. Many researchers focus on other intriguing materials such as g-C₃N₄-, SnO₂-, and ZnO-based heterostructured photocatalysts owing to their tunable bandgaps and photocatalytic efficiency [6, 8, 11].

For recent years, Ag₃PO₄ is being an intriguing candidate for photocatalytic degradation. Ag₃PO₄ possesses high photocatalytic efficiency and its quantum efficiency is nearly 90% [12]. In addition to that, the bandgap of Ag₃PO₄ is 2.34 eV, which makes it suitable for solar-light-driven photocatalysis. In this regard, Ag₃PO₄ can be undoubtedly an interesting candidate to be studied. Nevertheless, its higher solubility and poor stability hinder its vast application [9]. Ag₃PO₄ is vulnerable to photocorrosion. However, Ag₃PO₄-based heterogeneous composites have been gaining substantial attention due to their enhanced physical and chemical properties, which make them outstanding photocatalysts [13, 14]. Hydroxyapatite (HA) is the inorganic mineral phase of biological hard tissues and is abundant in nature. HA is a remarkable candidate in biomedical as well as environmental applications due to its exceptional biocompatibility, nontoxicity, structural flexibility, facile synthesis, and cost-effectiveness [15, 16]. HA is being tailored with tunable characteristics in a variety of applications owing to (1) the flexibility of the apatite structure that enables HA to host almost half of the elements in the periodic table, (2) an effortless ion-exchange process facilitates the replacement of Ca²⁺ ions in HA by a wide range of ions, and (3) substitution of both anions and cations is possible, i.e., PO₄³⁻ and OH⁻ functionalities in the apatite structure can be easily replaced by a variety of species like SeO₃²⁻, VO₄³⁻, SiO₄²⁻, CO₃², and halides [17–19]. Moreover, HA can be a prominent substrate for a photocatalyst like Ag₃PO₄ for the following features: (1) adsorption capability of the catalyst is very crucial in photocatalysis process and HA can serve this being a well-known adsorbent for inorganic and organic matters, (2) mass transfer limitations can be reduced, (3) hinders the photogenerated electron–hole recombination by effective separation, (4) heterostructures can easily be tailored, and (5) possesses excellent thermal stability, biodegradability, and nontoxicity [9, 15, 20].

In this work, HA_Ag₃PO₄ composites were synthesized in two steps in which neat hydroxyapatite was synthesized via chemical precipitation technique and Ag₃PO₄ nanoparticles were anchored on hydroxyapatite via a facile and at the same time very effective postsynthesis process, ion-exchange method [21]. Still, more studies are required in order to explore the behaviors of the photocatalyst for different dyes. In this work, the photocatalytic behavior of Ag₃PO₄ composite in the degradation of the commercial textile dye Sunfix Red (SR) S3B 150% for the first time was studied, and also the degradation under different experimental conditions was investigated. Moreover, the applicability of the prepared composites on other commercial dyes was examined.

2. Experimental

2.1. Materials

All the reagents of analytical grade were purchased and used without further purification. Calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O), diammonium hydrogen phosphate ((NH₄)₂HPO₄), silver nitrate (AgNO₃), and NH₄OH 25% were obtained from Merck, Germany.

2.2. Preparation of Neat HA

The neat HA was prepared by wet chemical precipitation technique reported elsewhere [22] which initially involves the preparation of 0.5 M calcium nitrate solution and 0.3 M diammonium hydrogen phosphate solution. Then, the phosphate solution was added to the calcium precursor at temperature 60°C. The pH of the reaction solution was maintained at 10.5 by adding sufficient amount of ammonium hydroxide and was stirred for 2 hr at 60°C followed by 24 hr aging. Later, the precipitate was filtered and washed many times with deionized water and ethanol. Subsequently, the precipitate was dried at 70°C for 24 hr. The chemical reaction is given as follows:

2.3. Preparation of Ag₃PO₄-Anchored HA (HA_Ag₃PO₄(x))

Ag₃PO₄-anchored HA was synthesized via in situ ion-exchange process reported elsewhere [14] and the schematic illustration is given in Figure 1. A hundred milliliters of AgNO₃ solution of appropriate concentration (0.02, 0.05, 0.08 M) was added dropwise into the aqueous dispersion of HA (3 g in 100 mL of deionized water). Subsequently, it was subjected to a vigorous and continuous stirring for 4 hr under dark condition. Then, the composite was separated by centrifugation, washed repeatedly with distilled water, and dried at 60°C in vacuum oven overnight. By this way, the nanocomposites were prepared with different AgNO₃ concentrations of 0.02, 0.05, and 0.08 M and named as HA_Ag₃PO₄(a), HA_Ag₃PO₄(b), and HA_Ag₃PO₄(c), respectively.

2.4. Characterization Techniques

The structural characterization was carried out through X-ray diffraction (XRD) pattern of the sample, which was recorded by a PANalytical X’Pert Pro X-ray diffractometer (λ = 1.5418 Å) at room temperature. The functional groups of the prepared samples were identified using Fourier transform infrared (FTIR) spectrometer (Perkin Elmer-Spectrum RX I). X-ray photoelectron spectra (XPS) was recorded by a PHI VersaProbe III photoelectron spectrometer and the surface chemical composition and valence state of the prepared samples were studied. The morphologies of samples were examined by JEOL-2100+ high-resolution transmission electron microscope. The optical properties of the samples were analyzed using the UV–vis spectrometer (Thermo Fisher Evolution 220). The concentration of the dye solutions was studied by Electra photometer.

2.5. Analysis of Photocatalytic Performance of the Prepared Composites

The photocatalytic behavior of the prepared samples was investigated through the photodegradation of the commercial organic dyes under sunlight. Initially, the anionic organic dye SR S3B 150% (C₄₄C_l2H₂₄N₁₄Na₆O₂₀S₆) was taken as a model and the photodegradation was evaluated under different experimental conditions such as Ag₃PO₄ concentration, dosage of photocatalyst, and concentration of the dye solution. In addition to these, the initial pH of the solution also influences the adsorption and degradation process and, hence, it is very essential for consideration [23]. Therefore, the photodegradation of the dye solution with different initial pH values was also examined. Later, the photocatalytic activity of the selected photocatalyst was also extended to other organic dyes, namely, Sunzol Black (SB) DN conc. (anionic), methylene blue (MB; cationic), rhodamine B (RhB; cationic), and cocktail (CT) of the aforementioned four different dyes. The required concentration of dye solutions was prepared and the initial pH of the dye solutions were adjusted with 2 mol/l NaOH solution or concentrated HNO₃. The appropriate amounts of catalysts were added to the dye solutions and were magnetically stirred for 1 hr in dark condition to attain adsorption and desorption equilibrium before photocatalytic studies. After irradiation with sunlight, the aliquots were collected in equal intervals and the concentration of the centrifuged aliquots was determined using UV–visible spectrometer. The decolorization rate of the dyes (D%) were calculated by the following equation:where C₀ and C were the initial and instantaneous concentrations of the dye solution, respectively.

The role of active radical species in the photocatalytic degradation process was also examined by applying scavengers. The scavengers such as isopropyl alcohol (IPA), disodium ethylenediaminetetraacetate (EDTA), and ascorbic acid were used to trap the hydroxyl (·OH), photogenerated holes (h⁺), and superoxide (·O₂⁻) radicals, respectively. The scavengers were added to the organic dye solutions at a concentration of 1 mmol/L before the addition of photocatalyst and subjected to the photocatalytic degradation as abovementioned.

3. Results and Discussion

3.1. Characterization of the Prepared Composites

3.1.1. XRD Analysis

The XRD patterns of as-prepared neat HA and HA_Ag₃PO₄(x) composites were presented in Figure 2. The XRD patterns of all the samples were indexed against HA (JCPDS# 009-0432) and Ag₃PO₄ (JCPDS Card#006-0505). The XRD patterns of HA_Ag₃PO₄(x), as shown in Figure 2, evidence the presence of Ag₃PO₄ in the background of HA. Besides, the intensities of the peaks representing Ag₃PO₄ get increased with the increase in AgNO₃ molar ratio; meanwhile, the characteristic HA peaks remain intact suggesting that the ion-exchange process proceeded by retaining the structure of HA. Further, it ascertains the composite nature of the HA_Ag₃PO₄(x).

3.1.2. FTIR Analysis

FTIR spectra of the prepared samples are depicted in Figure 3 and are exhibiting the signature bands of hydroxyapatite. The bending vibrational modes of PO₄³⁻ are identified at 566 and 610 cm⁻¹ and the absorption bands corresponding to 1,038–1,108 and 967 cm⁻¹ are ascribed to the stretching vibration modes of PO₄³⁻. The presence of absorption peaks at 1,389 and 1,645 cm⁻¹ approves the incorporation of carbonate ions in HA structure. The presence of hydroxyl group is confirmed by the absorption at 3,586 cm⁻¹ and the broad band around 3,425 cm⁻¹ is due to the absorbed water [24, 25]. The absorption regions (967 and 1,080 cm⁻¹) of asymmetric stretching of P–O–P associated with Ag₃PO₄ coincide with the signature phosphate bands of HA and no metal Ag-related bands in the observed FTIR spectra of the composites (b–d), as shown in Figure 3, were detected [26].

3.1.3. XPS Analysis

Neat HA and HA_Ag₃PO₄(c) were subjected to the XPS analysis and Figures 4 and 5 display their corresponding spectra. The wide range spectra of neat and composite HA exhibit the presence of the elements of hydroxyapatite and additionally Ag in the composite. The characteristic doublet at the binding energies 373.35 and 367.46 eV corresponding to Ag⁺ was evidenced for the sample HA_Ag₃PO₄(c) (Figure 5(d)), which strongly suggests the presence of Ag₃PO₄ in the composite [27, 28]. While examining the core level spectra of Ca and P in the samples HA and HA_Ag₃PO₄(c), there is no significant shift found as the corresponding binding energies to Ca 2p_3/2 and Ca 2p_1/2 were detected around 346.5 and 350 eV, and for P, it was observed around 132.5 eV [29]. The deconvolution of O 1s core spectra of HA_Ag₃PO₄(c) discloses three distinct peaks. The binding energy 529.5 eV is ascribed to the lattice oxygen of Ag₃PO₄, which is absent in that of neat HA. The other two peaks are of binding energies 530.9 and 532.6 eV. The earlier is assigned to the oxygen atoms in P=O and the later can be contributed by the oxygen from O–H [29].

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In addition, the photoelectron peaks corresponding to C 1s were also identified. It is quite common to get carbonated hydroxyapatite since the synthesis method in open air facilitates the incorporation of atmospheric carbonates in the HA structure. The photoelectron peaks around 284.5 and 286.1 eV are representing to C–C and C–O–C of the absorbed hydrocarbons, whereas 288.2 eV is ascribed to C=O bonds in carbonate [30].

3.1.4. TEM Analysis

The morphological features of the synthesized neat HA and HA-anchored Ag₃PO₄ composites are depicted in Figure 6. The neat HA prepared via simple wet precipitation method exhibited nanorod morphology (Figure 6(a)). While examining the TEM image of composite HA_Ag₃PO₄(a), the homogeneous distribution of Ag₃PO₄ spheres of size about 4 nm over the surface of hydroxyapatite is evidenced (Figure 6(d)). The TEM investigation was also extended to the composite HA_Ag₃PO₄(c) with the highest Ag⁺ concentration in the present study, which is presented in Figures 6(e) and 6(f). Interestingly, Ag₃PO₄ formations are found to be greater in number than that could be observed in the case of HA_Ag₃PO₄(a) (lower Ag⁺ concentration). The higher Ag⁺ concentration (0.08 M) facilitated a larger amount of nucleations over HA surface and also resulted in the reduction of Ag₃PO₄ particle size as indicated by blue arrows in Figure 6(e). Due to the decline in particle size, the surface area to volume ratio was expected to increase, which is a more favorable condition for photocatalysis [27]. Meanwhile, few larger sized Ag₃PO₄ spheres around 5 and 15 nm were also observed on HA as indicated by yellow arrows in Figure 6(e).

3.1.5. UV–Vis Diffuse Reflectance Spectroscopy

The UV–visible diffuse reflectance spectra (DRS) of pure HA and HA_Ag₃PO₄(x) are shown in Figure 7 and their bandgaps are depicted in Figure 8. Investigation of the bandgaps (Figure 8) gives the essential information about the photocatalytic behavior under visible light. The bandgap of neat HA was found to be 3.4 eV, whereas the composites (HA_Ag₃PO₄(x)) exhibited bandgap energy of 2.34 eV. The narrowed bandgap (2.34 eV) of the composites is due to the presence of Ag₃PO₄ which is well matched with the reported values of Ag₃PO₄ [28] and enables the photocatalyst to perform under visible light.

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In addition to the bandgap, the valence band and conduction band edge potentials were also estimated by the following empirical formula:where E_VB and E_CB are the valence band and conduction band edge potentials, respectively; χ is Mulliken electronegativity of Ag₃PO₄ which is the geometric mean of the electronegativity of the constituent atoms and it was calculated to be 5.96 eV; E^c represents the energy of free electrons on the hydrogen scale (−4.5 eV); and E_g is the bandgap energy of Ag₃PO₄. Thus, calculated VB and CB potentials of Ag₃PO₄ are 2.63 and 0.29 eV, respectively.

3.2. Photocatalytic Degradation of Organic Dyes

The photocatalytic degradation of all the selected organic dyes in the presence of the prepared photocatalysts were analyzed after 60 min adsorption and considered as 0 min for photocatalysis process in sunlight. First, photodegradation of the anionic dye SR S3B 150% was carried out under different conditions such as dye concentration, photocatalyst concentration, and pH of the dye solution, and their influence on the degradation of the dye was investigated. In addition, blank dye solution in the absence of photocatalyst was also examined as a control. For the better investigation of the photocatalytic behavior of the prepared samples, zero-order, first-order, and second-order kinetic studies were carried out and the rate constants were determined by the following equations:

zero-order kinetic equation:first-order kinetic equation:second-order kinetic equation:where k₀, k₁, and k₂ are the rate constants of zero-, first-, and second-order kinetics, respectively; C₀ and C are the concentrations of the dye solution before and after irradiation at time, t.

3.2.1. Effect of Ag₃PO₄ Loading on HA

The adsorption under dark condition and photodegradation of the model dye pollutant SR S3B 150% (20 ppm) under visible light (sunlight) at a fixed dosage concentration of 0.5 g L⁻¹ were studied and presented in Figure 9. It is evidenced that the adsorption capacity of the photocatalyst is increased with Ag₃PO₄ concentration. The blank dye solution and with HA did not show a significant degradation under sunlight irradiation, whereas the composites HA_Ag₃PO₄(x) exhibited photocatalytic activity with an increasing trend with increase in Ag₃PO₄ concentration with rate constants 0.00515, 0.02178, and 0.07168 min⁻¹, respectively (Table 1). The composite HA_Ag₃PO₄(c) has the highest rate constant compared with other photocatalysts used in this study and, hence, HA_Ag₃PO₄(c) was selected for further photodegradation investigations. The highest performance of HA_Ag₃PO₄(c) can be explicitly understood that the higher amount of Ag₃PO₄ gives rise to higher active sites, which leads to higher production of reactive radical species. This is also confirmed by TEM image of HA_Ag₃PO₄(c) (Figure 6(e)), which revealed the larger number of Ag₃PO₄ particles on HA and also it is in agreement with XRD result. Moreover, it is very clear from TEM result that the Ag₃PO₄ particles formed by ion substitution on HA substrate have a declined particle size compared with the lower Ag⁺ concentration, which in turn enhances the active surface area of the material. The surface area of the photocatalytic material is one of the very important parameters to influence photocatalysis [27].

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3.2.2. Effect of Dosage of HA_Ag₃PO₄(c)

From the above findings, it is evident that the composite HA_Ag₃PO₄(c) performed an excellent photocatalytic activity and, hence, HA_Ag₃PO₄(c) was taken for further studies. The dependence of HA_Ag₃PO₄(c) on dosage concentration in the photocatalytic degradation of SR S3B 150% was examined. The photocatalyst concentration range was selected from 0.25 to 2 g L⁻¹ at the dye concentration of 20 ppm and their photocatalytic performances were examined for 1 hr, as depicted in Figure 10. As the photocatalyst quantity increased, an increasing trend of degradation of the dyes can be noticed. It is general to have an increasing photodegradation as the number of active sites on the surfaces of the photocatalyst increases. Consequently, it gives rise to large number of reactive radical species. At the end of the photocatalysis (60 min irradiation), the lowest concentration of HA_Ag₃PO₄(c) (0.25 g L⁻¹) degraded the dye to 53%, while the other concentrations 0.5, 1 and 1.5 g L⁻¹ degraded the dye completely at 45, 30, and 15 min, respectively. The rate constants obtained from ln (C₀/C) versus time plot for the concentrations 0.25, 0.5, 1, and 1.5 are 0.009, 0.034, 0.046, and 0.0924, respectively. In the same time, the dye was completely adsorbed at the maximum concentration of 2 g L⁻¹ and the sample dye solution became turbid. Hence, the concentration of HA_Ag₃PO₄(c) is limited to 1.5 g L⁻¹.

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3.2.3. Effect of Initial Concentration of the Dye Solution

The impact of initial concentration of the dye solution on photodegradation of SR S3B 150% was studied in the range from 10 to 50 ppm at a fixed photocatalyst concentration 0.5 g L⁻¹, which is presented in Figure 11. It was observed that the percentage of degradation was declined with the increase in dye concentration. The dye solution of concentrations 10 ppm at 30 min were undergone degradation of 100% while the rest of the concentrations 20–50 ppm showed a degradation of 66.6%, 53.8%, 47.2%, and 44%, respectively, at 30 min. A declining trend in the rate constants for the degradation process of increasing concentration was noticed (10, 20, 30, 40, and 50 ppm—0.04621, 0.03662, 0.02613, 0.02246, and 0.01921 min⁻¹, respectively). This can be clarified by two ways as follows: one reason can be the limitation of the number of active sites on the photocatalyst surface which may be insufficient to degrade the higher concentrations and the other can be explained in terms of amount of adsorption of dyes. As the increase in dye concentration leads to higher amount of dye to be adsorbed on the catalyst surface, it shields the catalyst from the radiation and hinders the photogeneration of the electron–hole pair which is responsible for the degradation of the dye [23, 31, 32].

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3.2.4. Effect of Initial pH of the Dye Solution

The initial pH of the polluted solution is a crucial influencing factor in controlling the photodegradation process. The pH affects the surface charge of the catalyst, pollutants adsorption onto the catalyst, and the valence band oxidation potential [33, 34]. The photocatalytic behavior of the selected photocatalyst, in the degradation of SR S3B 150% under a broad range of pH conditions from 2 to 10 with two-step variation, was examined at a fixed irradiation time of 15 min. The results revealed that acidic condition favored the enhanced photodegradation than neutral condition. The maximum photodegradation of the organic dye SR was observed to be 96.8% at pH 4 and also at pH 2 and 6 comparable degradation efficiency (93.5% for both) were noticed, but very low degradation efficiencies were found at higher pH values. As shown in Figure 12(a), it is evidenced that the higher adsorption facilitated higher photodegradation. At acidic conditions, Ag₃PO₄ becomes positively charged and the higher adsorption of the anionic dye SR occurred due to the electrostatic attraction between the pollutant and catalyst. In the same time, at basic conditions, the deprotonated OH⁻ groups over the catalyst surface hinder the adsorption of the anionic dye by electrostatic repulsion [35]. Even though the higher pH resulted in lower adsorption, a faster degradation was observed at pH 10. This could be due to the lowering of the oxidation potential of the dye and eventually the dye can be degraded quickly [36].

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3.2.5. Quenching Test

The free radicals employed in the photocatalytic process, namely, such as hydroxyl (·OH), superoxide (·O₂⁻), and photogenerated holes (h⁺) were trapped by IPA, ascorbic acid, and EDTA, respectively, under similar conditions. The scavengers were taken in the concentration of 1 mmol/L and the concentrations of the dye and the photocatalyst were fixed to 20 ppm and 0.5 g L⁻¹, respectively, and irradiated by direct sunlight for 60 min and the results are given in Figure 13.

From the free radicals trapping test, it can be noted that the photocatalytic degradation was decreased from 100% to 82% with the addition of IPA which reveals that role played by hydroxyl radicals in the degradation of the dye SR is only to a small extend, while the other radical trappers EDTA and ascorbic acid deteriorated the degradation of the dye significantly (46% and 35%, respectively). It is clear that the superoxide radicals and holes took part actively in the photocatalysis process.

3.2.6. Degradation of Other Dyes

The study of photocatalytic behavior of the photocatalyst HA_Ag₃PO₄(c) on some other anionic and cationic dyes was also extended. Anionic dyes such as SR S3B 150% and SB DN conc., the cationic dyes such as MB and RhB, and cocktail of all the four dyes were subjected to photodegradation by HA_Ag₃PO₄(c) under similar conditions and presented in Figure 14. The photocatalytic experiments were carried out for 150 min at 20 ppm dye concentration and photocatalyst concentration of 1 g L⁻¹. All the dyes showed different degradation rate, which is clear from Figure 14. SR and SB were decomposed completely in 30 and 60 min, respectively, while MB and RhB were decolorized to 98.16% and 78.67%, respectively, at the irradiation time 1 hr 45 min. Meanwhile, the degradation of cocktail dye solution was found to be 96.25% in 1 hr 45 min. The rate constants of the degradation of the different dyes are observed in the order k_SR > k_SB > k_MB > k_CT > k_RhB which are depicted in Table 2.

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4. Possible Photocatalytic Mechanism in the Degradation of Sunfix Red S3B 150%

From the above findings, it can be noted that the concentration of AgNO₃ precursor taken for the formation of HA_Ag₃PO₄ composite influenced the photocatalytic process in two ways. First, as the concentration of AgNO₃ increased from 0.02 to 0.08 M, it showed an increasing order of photocatalysis due to the increased number of active sites available. Meanwhile, it can be seen from the TEM results that the higher concentration of the Ag⁺ precursor also resulted in reduction of particle size which led to enhanced photodegradation of dyes owing to increase in surface-to-volume ratio. In the process of photocatalytic degradation, the photogenerated electron–hole recombination inhibits the photocatalysis [27]. Hence, proper charge separation is required to make them available to carry out the redox process. Many results reported that heterostructures can inhibit electron–hole recombination through creating a proper transfer channel for the photogenerated charges [27, 37]. In the present work, the adopted synthesis method has resulted in good interfacial bonding between HA and Ag₃PO₄ which has been clearly noticed from the TEM result (Figure 6(f)). This channel may facilitate an easy transfer of photogenerated electrons e_CB⁻ from Ag₃PO₄ to hydroxyapatite being a good electron acceptor [38].

Based on the obtained results, the possible mechanism of photocatalytic degradation is proposed (Figure 15). In the presence of direct sunlight, the photocatalyst is excited and electron–hole pairs are generated. The majority of photogenerated holes (h_VB⁺) in the valence band directly involve in the oxidation of the organic dye, while some of them generate some ·OH, which may contribute a weak photocatalysis, which is confirmed from the quenching test. It is also evident from the estimated value of conduction band potential (0.29 eV) of Ag₃PO₄ that it is more positive than O₂/·O₂⁻ (−0.33 eV vs. NHE) and hence the photoexcited electrons (e_CB⁻) could not reduce the dissolved O₂ to superoxide radicals (·O₂⁻). This may result in the reduction of lattice Ag⁺ to Ag metal which may eventually suppress the availability of photogenerated electrons for photocatalytic process [36]. However, hydroxyapatite in the composite captures the photogenerated electrons and makes them available for photocatalysis. Moreover, the transferred photogenerated electrons reduce the adsorbed O₂ to superoxide radicals (·O₂⁻) which actively take part in the photocatalytic degradation of the organic dye under direct sunlight.

4.1. Reusability

The reusability of the prepared composite HA_Ag₃PO₄(c) was examined for four cycles and presented in Figure 16. The sample after every cycle of photocatalytic degradation of the dye was regenerated by washing three times with DI water and was dried at 60°C in a vacuum oven and used for the next cycle. The performance of the prepared composite declined gradually after each cycle and the degradation efficiency reduced to 52% from 100% after the fourth cycle of degradation. The photocatalyst after the fourth run was subjected to XRD analysis to investigate the stability and presented in Figure 17. The presence of the peak at 38° is corresponding to pure Ag (111) plane which reveals that the photocatalyst underwent slight photocorrosion after four cycles. The removal efficiency of the prepared sample was also compared with the previous results and presented in Table 3.

5. Conclusion

Ag₃PO₄-anchored hydroxyapatite was synthesized via a facile in situ ion-exchange method. From the results, it was found that the particle size of Ag₃PO₄ can be tuned by the concentration of AgNO₃ precursor solution. The photocatalytic performance on an anionic dye SR S3B 150% under visible light was carried out at different experimental conditions and found satisfactory results. The photodegradation studies were also extended for other organic dyes such as SB, MB, RhB, and cocktail of all four dyes. The studies revealed that the synthesized composites can be a proficient photocatalyst for the degradation of studied organic dyes.

Data Availability

All data generated or analyzed during this study are included in the published article.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Acknowledgments

S. Dhatchayani gratefully acknowledges UGC, New Delhi, for awarding UGC-BSR fellowship (F. No. 25-1/2014-15 (BSR)/7-14/2007(BSR)/13.03.2015) and the authors greatly acknowledge Rusa–Phase 2.0 grant sanctioned vide letter no. F. 24-51/2014-U, Policy (TNMulti-Gen), Department of Education, Government of India, Dt.09.10.2018., UGC-SAP, DST-FIST, and DST-PURSE for financial support to carry out this work. We thank Mr. N. Pandi and Mrs. P. Kanchana for the supply of dyes for the present studies and also extend our gratitude to USIC, Alagappa Uuniversity for rendering instruments for characterizations.

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Copyright © 2023 Surendran Dhatchayani et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Journal of Nanomaterials

Eco-friendly Waste-based Nanocatalyst Materials in Energy, Environment and Biological Applications

Tailoring of Ag3PO4-Anchored Hydroxyapatite Nanophotocatalyst with Tunable Particle Size by a Facile Ion-Exchange Method for Organic Textile Dyes Photodegradation

Abstract

1. Introduction

2. Experimental

2.1. Materials

2.2. Preparation of Neat HA

2.3. Preparation of Ag3PO4-Anchored HA (HA_Ag3PO4(x))

2.4. Characterization Techniques

2.5. Analysis of Photocatalytic Performance of the Prepared Composites

3. Results and Discussion

3.1. Characterization of the Prepared Composites

3.1.1. XRD Analysis

3.1.2. FTIR Analysis

3.1.3. XPS Analysis

3.1.4. TEM Analysis

3.1.5. UV–Vis Diffuse Reflectance Spectroscopy

3.2. Photocatalytic Degradation of Organic Dyes

3.2.1. Effect of Ag3PO4 Loading on HA

3.2.2. Effect of Dosage of HA_Ag3PO4(c)

3.2.3. Effect of Initial Concentration of the Dye Solution

3.2.4. Effect of Initial pH of the Dye Solution

3.2.5. Quenching Test

3.2.6. Degradation of Other Dyes

4. Possible Photocatalytic Mechanism in the Degradation of Sunfix Red S3B 150%

4.1. Reusability

5. Conclusion

Data Availability

Conflicts of Interest

Acknowledgments

References

Copyright

Tailoring of Ag₃PO₄-Anchored Hydroxyapatite Nanophotocatalyst with Tunable Particle Size by a Facile Ion-Exchange Method for Organic Textile Dyes Photodegradation

2.3. Preparation of Ag₃PO₄-Anchored HA (HA_Ag₃PO₄(x))

3.2.1. Effect of Ag₃PO₄ Loading on HA

3.2.2. Effect of Dosage of HA_Ag₃PO₄(c)