Synthesis of TiO₂–Ag₃PO₄ photocatalyst material with high adsorption capacity and photocatalytic activity: application in the removal of dyes and pesticides

Abstract

The photocatalytic activity of TiO₂ can be enhanced by coupling it with other semiconductors and the semiconductor composites may find useful application in water treatment technologies. TiO₂–Ag₃PO₄ composites were synthesized and characterized with XRD, SEM-EDX and DRS. The synthesized TiO₂–Ag₃PO₄ showed high photocatalytic activity in the presence UV-vis light on rhodamine B, methylene blue and the pesticides imidacloprid, atrazine and pyrimethanil. LC-MS analysis of the photodegraded pyrimethanil led to the identification of hydroxylated and aliphatic derivatives of pyrimethanil. The photocatalytic activity of the coupled semiconductor was higher than that of the bare TiO₂ and Ag₃PO₄ and this was attributed to the unique band matching between TiO₂ and Ag₃PO₄ which resulted in efficient charge separation and subsequent reduction in the electron–hole recombination. In addition, the synthesized TiO₂–Ag₃PO₄ showed strong adsorption for water soluble dyes implying that TiO₂–Ag₃PO₄ can remove pollutants through photocatalysis and adsorption. The results from the study showed the potential application of TiO₂–Ag₃PO₄ composite in water treatment technologies.

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1 Introduction

Increased population and industrial activities have resulted in industrial effluent generation and pollution of water bodies, harmful to humans and aquatic life especially when they contain substances such as dyes, pesticides, heavy metals and pharmaceutical wastes.¹

The world production of dyes has increased dramatically over the past years and these azo dyes, reactive dyes, solvent dyes for instance are used by the textile, food, adhesive, cosmetics, arts, construction, paints, glass and ceramic industries extensively.² The release of these colored organic substances into our environment is a source of aesthetic pollution and is detrimental to the environmental ecosystems. In addition, large concentrations of organic dyes in the aquatic environment may be toxic to aquatic species and disulphonated azo dyes has been reported to be a major source of problem in drinking water plants globally.³

In addition there is high demand for pesticides globally as a result of increasing agricultural activities and the toxins in the pesticides are leached into surface and ground water sources which are detrimental to human health and aquatic life.⁴ The alarming impact of these activities on the environment has prompted the European Union to set standards under the European Directive 2000/60/EC that establishes maximum concentrations for pesticides and related harmful chemical substances in drinking waters to safeguard its detrimental effects on humans.⁵

High chemical stability and low biodegradability of these organic pollutants have necessitated the search for suitable treatment methods that can easily break down inherent chemical components. Techniques such as adsorption, biodegradation, ozonation and photo-Fenton processes have been adopted in treating dyes and pesticides pollutants.⁶ However, these methods have been reported to be ineffective in the treatment of polluted water bodies.⁷ Recent studies have been focused on the use of advance oxidation processes such as photocatalysis for the removal of dyes and pesticides from wastewaters,⁸ degradation of crude oil fractions,⁹ and for aquatic disinfection.¹⁰

Photocatalysis involves the generation of reactive oxygen species (ROS) in the presence of sunlight, semiconductor, dissolved oxygen and water. These ROS are able to oxidize and mineralize organic contaminants into carbon dioxide and inorganic anions.¹¹ Hence, semiconductor based photocatalysts have received enormous attention because of their potential in solving some of the most challenging environmental problems. One of the most widely investigated photocatalyst is TiO₂. Though TiO₂ is chemically stable, its wide band gap energy limits its efficient utilization in sunlight irradiation. That is TiO₂ is active in the UV region of the electromagnetic spectrum which makes up only ca. 4–5% of the electromagnetic spectrum and this limits its practical application. Developing semiconductor materials that are very sensitive to visible light and can be effectively utilized in practical applications has become the focus of many researchers. Different visible light active photocatalysts such as Au/TiO₂,¹² C–TiO₂/g-C₃N₄,¹³ YFeO₃,¹⁴ defect ZnO nanorods¹⁵ and ZnO/CuO¹⁶ have been developed by researchers. These semiconductor photocatalyst may be decorated with bimetallic nanoparticle cocatalyst to enhance their degradation efficiency.¹⁷ In addition, 3D graphene based photocatalyst have been developed and utilized in environmental pollutant degradation.¹⁸ Ag₃PO₄ with extremely high visible light photocatalytic activity has also been reported.¹⁹

Photocorrosion which involves the conversion of Ag⁺ to Ag⁰ in the absence of an electron acceptor has been reported as a major drawback of Ag₃PO₄ photocatalyst. This occurs as a result of the solubility of Ag₃PO₄ in aqueous solution.^19a,20 In addition to using Ag₃PO₄ as a stand-alone photocatalyst, several composite photocatalyst have been developed by combining Ag₃PO₄ with AgI,²¹ gC₃N₄,²² carbon quantum dots,²³ SrTiO₃ (ref. 24) and TiO₂.²⁵ TiO₂–Ag₃PO₄ has been reported to be the most efficient among the composite photocatalyst formed with Ag₃PO₄.²⁶ A photocatalytic system made up of carbon fiber cloth as a porous substrate and Ag₃PO₄/TiO₂ as a photocatalyst with a rhodamine B dye removal efficiency of ∼99.5% has been reported.²⁷ Graphene oxide (GO) embedded ternary heterostructure nanocomposites of N–TiO₂/Ag₃PO₄@GO with different weight ratios of N–TiO₂ to Ag₃PO₄ has also been developed.²⁸ It should however be mentioned that, to the best of the authors knowledge, efficiency of all the reported TiO₂–Ag₃PO₄ photocatalyst was examined using water soluble dyes and phenols,²⁹ and none of the articles reported a detailed adsorption kinetics and equilibrium studies. A detailed adsorption equilibrium and kinetic studies is required to understand the adsorption mechanism of Ag₃PO₄–TiO₂ towards organic molecules.³⁰

In this study, a one pot method was used to synthesize Ag₃PO₄–TiO₂ photocatalyst. The synthesized photocatalyst was characterized with XRD, SEM-EDX, and DRS. The photodegradation efficiency was examined using rhodamine B dye, methylene blue dye and three pesticides; atrazine, imidacloprid and pyrimethanil. Detailed adsorption kinetic and equilibrium studies was conducted on the Ag₃PO₄–TiO₂ using methylene blue dye. The photodegradation by-products obtained after subjecting pyrimethanil to photocatalysis were identified using LC-MS.

2 Experimental procedure

2.1 Materials

All chemical reagents used in this study were analytical grade and were purchased from Sigma Aldrich, U.K. The chemical reagents were used without further purification. Deionized water was used throughout this study.

2.2 Synthesis of Ag₃PO₄–TiO₂ nanocomposite

In a typical synthesis, a calculated amount of TiO₂ (P25) was dispersed in 150 mL of 0.02 M AgNO₃ by sonicating for 30 minutes. 122.5 mL of 0.2 M Na₂HPO₄ was then added dropwise while stirring. The resulting bright yellow solution was continuously stirred for 30 minutes and then centrifuged to collect the bright yellow precipitate. The collected precipitate was washed repeatedly (three cycles) with deionized water and dried at 70 °C to obtain Ag₃PO₄–TiO₂ nanocomposite. Different amounts of TiO₂ were used in the synthesis to obtain samples with 10 wt%, 25 wt%, 50 wt% and 75 wt% TiO₂. A similar procedure was used to obtain Ag₃PO₄ but without the addition of TiO₂. The synthesis scheme of the Ag₃PO₄–TiO₂ nanocomposite is provided in Fig. 1 below.

Fig. 1 Synthesis scheme of the Ag₃PO₄–TiO₂ nanocomposite.

2.3 Product characterization

2.3.1 X-ray diffraction (XRD). The X-ray diffraction patterns were obtained with Bruker D8 advanced focus diffractometer fitted with position sensitive detector (LynxEye) and standard detector. Cu-Kα radiation (λ = 0.15405 nm) and a 2theta angular range of 10–80° were used.

2.3.2 Scanning electron microscopy/energy-dispersive X-ray spectroscopy. The morphology of the synthesized nanocomposites was examined with scanning electron microscopy (FEI Nova NanoSem) connected to EDX acquisition detector. The elemental composition was determined through EDX.

2.3.3 Diffuse reflectance spectroscopy (DRS). DRS was conducted using Ocean Optics USB-4000 spectrometer with a dedicated reflectance probe. Illumination was supplied with a Deuterium/Halogen source across the UV-VIS range. The synthesized nanocomposites are compressed into a flat film between glass microscope slides prior to measurement and a commercial PTFE reflectance standard was sued as reflectance calibration.

2.3.4 Adsorption studies. The adsorption capability of the synthesized nanocomposite was examined using methylene blue dye. 100 mg of Ag₃PO₄–TiO₂ was added to 200 mL aqueous solution of methylene blue (5 mg L⁻¹). Aliquots of the methylene blue solution were taken at specified time intervals. The sampled solution was centrifuged at 6000 rpm for 5 minutes to remove the powdered nanocomposite. The concentration of methylene blue was obtained by measuring the absorbance at 655 nm wavelength using Ocean Optics 4000 USB modular UV-VIS spectrometer.

2.3.5 Photocatalytic methylene blue, rhodamine blue and pesticide degradation. The photocatalytic activity of the synthesized Ag₃PO₄–TiO₂ was examined using rhodamine B (6 mg L⁻¹) and methylene blue (8 mg L⁻¹) dyes and pesticides (pyrimethanil, imidacloprid and atrazine). In a typical photocatalytic degradation of rhodamine B dye, 200 mL of rhodamine B solution in deionized water was used with a photocatalyst suspension concentration of 0.5 g L⁻¹. A jacketed glass reactor with a quartz tube immersion well was used with illumination from 300 W Tungsten Halogen lamp. For each of the experiments, the solution was stirred in the dark for 30 minutes (24 hours for pesticide solutions) to attain an adsorption–desorption equilibrium for illumination. 2 mL aliquots were taken at specific time intervals and centrifuged at 6000 rpm for 5 minutes to remove the powdered photocatalyst. The dye concentration was quantified using Ocean Optics 4000 USB modular UV-vis spectrometer with an absorbance values taken at 554 nm. A similar procedure was used for the photocatalytic degradation of the methylene blue and pesticides. However, the absorbance values were taken at 665 nm for methylene blue, 269 nm (pyrimethanil), 270 nm (imidacloprid), and 222 nm (atrazine). The degraded pesticide solution obtained from pyrimethanil was analyzed with LC-MS to identify photodegraded products.

3 Results and discussion

3.1 Characterization of Ag₃PO₄–TiO₂

The XRD patterns of the pristine P25 TiO₂, Ag₃PO₄, and Ag₃PO₄-50 wt% TiO₂ are presented in Fig. 2. The XRD pattern for the Ag₃PO₄ showed well defined crystalline Ag₃PO₄ with sharp and intense diffraction peaks that can be indexed to the body-centered crystal structure (JCPDS no. 06-0505).

Fig. 2 XRD patterns for (A) Ag₃PO₄, TiO₂ and Ag₃PO₄-50 wt% TiO₂, (B) enlarged XRD pattern for TiO₂ [O = Ag₃PO₄, ▭ = TiO₂].

The dominant peaks at 33.4° and 36.6° correspond to the (210) and (211) crystallographic planes,³¹ respectively. On the other hand, TiO₂ showed an intense peak at 25.5° which represents the (101) crystallographic plane and can be indexed to the anatase phase (JCPDS no. 21-1272). With the “rectangles” representing the anatase phase present in TiO₂ and the circles representing the cubic structure of Ag₃PO₄, the prepared Ag₃PO₄–TiO₂ composite has all the crystallographic planes present in the crystalline cubic structure of Ag₃PO₄ and the (101) plane of the anatase phase of TiO₂. The as-prepared Ag₃PO₄–TiO₂ is therefore expected to exhibit high photocatalytic activity. Though the Ag₃PO₄–TiO₂ used for the analysis contained 50 wt% TiO₂, the diffraction peaks of the TiO₂ is weak. This can be ascribed to the relatively low crystallinity of TiO₂ when compared with that of Ag₃PO₄.

The morphology of the synthesized Ag₃PO₄ and Ag₃PO₄–TiO₂ photocatalyst were examined with SEM and the images presented in Fig. 3. The pristine Ag₃PO₄ is irregularly shaped spherical particles with diameter of ca. 100–400 nm. The P25 used for this study has a much smaller diameter of ca. 20 nm. The SEM images showed the agglomeration of the TiO₂ particles. The particle sizes of the Ag₃PO₄ and TiO₂ in the Ag₃PO₄–TiO₂ composite (Fig. 3 B–D) is similar to the pure Ag₃PO₄ and TiO₂. It can be seen from Fig. 3B to D that the TiO₂ covered the surface of the Ag₃PO₄ creating a heterojunction. The extent of the surface coverage is dependent on the wt% of the TiO₂ used for the synthesis. This coverage is expected to influence the photocatalytic activity. Positively charged ions adsorbs strongly to the surface of TiO₂.^29a,32 Since the P25 TiO₂ was first dispersed in AgNO₃, the Ag⁺ ions adsorbed onto the surface of the TiO₂. The reaction between the Ag⁺ and the PO₄³⁻ occurred on the surface of the TiO₂ when the Na₂HPO₄ was added, resulting in the formation of the TiO₂–Ag₃PO₄ composite. The SEM-EDX (Fig. 3E and F) spectrum confirms the presence of Ti, O, Ag and P. The elemental mapping showed that the elements Ti and O were evenly distributed on the surface of the Ag₃PO₄.

Fig. 3 SEM images (A) Ag₃PO₄, (B) Ag₃PO₄-25 wt% TiO₂, (C) Ag₃PO₄-50 wt% TiO₂ and (D) Ag₃PO₄-75 wt% TiO₂; (E) EDX mapping for Ag₃PO₄-50 wt% TiO₂, (F) EDX spectrum for Ag₃PO₄-50 wt% TiO₂.

The UV-vis diffused reflectance spectra of the product with different wt% of TiO₂ are shown in Fig. 4. The P25 TiO₂ only absorbs light in the UV region of the electromagnetic spectrum with wavelength lower than approximately 390 nm. On the other hand, the pure Ag₃PO₄ has intense light absorption in the visible light region of the electromagnetic spectrum. The shape of the DRS spectra of the composite is a combination of that of Ag₃PO₄ and TiO₂ further proving the absence of any chemical interaction between TiO₂ and Ag₃PO₄. This agrees with observation made by Taheri et al.³³ The composite Ag₃PO₄–TiO₂ showed strong absorption in the visible light region due to the excellent visible light absorption by Ag₃PO₄. The composite Ag₃PO₄–TiO₂ can therefore harvest more visible light and result in an improved photocatalytic activity. The optical band gap of the photocatalysts synthesized were estimated from the DRS spectra using the Kubelka–Munk model³⁴ and are presented in Fig. 5. The optical band gap of the TiO₂ and Ag₃PO₄ were estimated to be 3.33 and 2.47 eV, respectively. The composite samples are characterized by two absorption thresholds, one corresponding to the TiO₂ and the other, Ag₃PO₄. A slight variation in the two absorption thresholds were observed upon varying the weight percent of the TiO₂ used for the synthesis. The band gap for the absorption threshold corresponding to TiO₂ decreased with decreasing wt% of TiO₂ in the TiO₂–Ag₃PO₄ composite as can be seen in Fig. 5.

Fig. 4 UV-vis DRS spectra of Ag₃PO₄, TiO₂ and the various formulations of Ag₃PO₄–TiO₂.

Fig. 5 Optical band gap estimation using Kubelka–Munk model for (A) TiO₂, (B) Ag₃PO₄, (C) Ag₃PO₄-10 wt% TiO₂, (D) Ag₃PO₄-25 wt% TiO₂, (E) Ag₃PO₄-50 wt% TiO₂ and (F) Ag₃PO₄-75 wt% TiO₂.

3.2 Photodegradation of rhodamine B, methylene blue and pesticides

Fig. 6A shows the photocatalytic performance of rhodamine B solution (200 mL, 6 mg L⁻¹) over Ag₃PO₄, TiO₂ and Ag₃PO₄–TiO₂ composites at a photocatalyst concentration of 0.5 g L⁻¹ under UV-vis light irradiation. Before irradiation, the dye-photocatalyst solution was stirred in the dark for 30 minutes to achieve adsorption–desorption equilibrium. The percentages of rhodamine B adsorbed by Ag₃PO₄, TiO₂ and Ag₃PO₄–TiO₂ composite were 14.5, 18.8 and 12.0 wt%, respectively. The wt% of the rhodamine B adsorbed by the various formulations of the composite material was ca. 12%. The data obtained from the photocatalysis was fitted with a pseudo first order kinetics and the rate constants (k) calculated (Fig. 6B). Under UV-vis light irradiation, ca. 9.8% of rhodamine B (k = 0.0153 min⁻¹) was degraded by TiO₂ within the time (6 min) frame that the photocatalysis analysis was carried out. On the other hand, ca. 66% of the rhodamine concentration was degraded by Ag₃PO₄ within 6 min (k = 0.168 min⁻¹). The rate at which the concentration of the dye solution degraded by the composite (Ag₃PO₄–TiO₂) decreased was significantly higher than that of Ag₃PO₄ and TiO₂. For the same time period (6 min), the degradation efficiency obtained from the composites were 90, 98, 96 and 79% for Ag₃PO₄-10 wt% TiO₂ (k = 0.3571 min⁻¹), Ag₃PO₄-25 wt% TiO₂ (K = 0.5406 min⁻¹), Ag₃PO₄-50 wt% TiO₂ (K = 0.5069 min⁻¹) and Ag₃PO₄-75 wt% TiO₂ (K = 0.2438 min⁻¹), respectively. These values indicate that Ag₃PO₄ and TiO₂ act synergistically to enhance photodegradation. This is so because the degradation efficiency and the rate constants of the composite Ag₃PO₄–TiO₂ were significantly higher than that of the individual components. The synergy was highest at 25 wt% TiO₂. A similar observation was reported by Taheri et al.³³ This observation can be attributed to the formation of a heterojunction between the TiO₂ and the Ag₃PO₄.³³ In addition, the photocatalytic activity is also affected by the extend of surface coverage. To obtain maximum photocatalytic activity, the extend at which the Ag₃PO₄ surface is covered by TiO₂ should be minimized. The uncovered surface of Ag₃PO₄ is utilized for maximum absorption of visible light.

Fig. 6 (A) Photocatalytic activities of Ag₃PO₄, TiO₂ and Ag₃PO₄–TiO₂ (10, 25, 50, and 75 wt% of TiO₂) and (B) rate constants of Ag₃PO₄(A), TiO₂ (T) and Ag₃PO₄–TiO₂ (A-wt% T) on 6 mg L⁻¹ rhodamine B dye.

The degradation of methylene blue (200 mL, 8 mg L⁻¹, 0.5 g L⁻¹ catalyst concentration) was also examined under UV-vis light irradiation and the results shown in Fig. 7. After the adsorption–desorption equilibrium was attained, 8.9 (Ag₃PO₄), 16.7 (TiO₂), 31.0 (Ag₃PO₄-10 wt% TiO₂), 66.4 (Ag₃PO₄-25 wt% TiO₂), 71.8 (Ag₃PO₄-50 wt% TiO₂) and 79.6% (Ag₃PO₄-75 wt% TiO₂) of the methylene blue dye was removed by adsorption. This informed the authors to conduct a detailed adsorption kinetics and equilibrium test. The photocatalytic activity of TiO₂ on methylene blue was low (16.7%, k = 0.0175 min⁻¹ within 4 min). Within the same time frame, Ag₃PO₄ recorded a photocatalytic efficiency of 79% with a rate constant of 0.3573 min⁻¹. From Fig. 7, it is observed that the Ag₃PO₄–TiO₂ composite is much more active photocatalytically than Ag₃PO₄ and TiO₂. The photocatalytic efficiency and the rate constants were 96.3% and 0.7338 min⁻¹, 99.1% and 0.9010 min⁻¹, 97.7% and 0.7069 min⁻¹ and, 94.8% and 0.3743 min⁻¹, for Ag₃PO₄-10 wt% TiO₂, Ag₃PO₄-25 wt% TiO₂, Ag₃PO₄-50 wt% TiO₂ and Ag₃PO₄-75 wt% TiO₂, respectively. Again, the Ag₃PO₄-25 wt% TiO₂ recorded the highest photocatalytic efficiency and rate constant.

Fig. 7 (A) Photocatalytic activities of Ag₃PO₄, TiO₂ and Ag₃PO₄–TiO₂ (10, 25, 50, and 75 wt% of TiO₂) and (B) rate constants of Ag₃PO₄ (A), TiO₂ (T) and Ag₃PO₄–TiO₂ (A-wt% T) on 8 mg L⁻¹ methylene blue dye.

The stability of the synthesized nanocomposite was examine using Ag₃PO₄-50 wt% TiO₂ and rhodamine B. The results for the stability/reusability of the nanocomposite is presented in Fig. 8. After five cycles of the photodegradation test, the degradation efficiency decreased from 96% to 85%. The developed Ag₃PO₄–TiO₂ nanocomposite is therefore fairly stable under the experimental conditions used in this study.

Fig. 8 The reusability of Ag₃PO₄–TiO₂ nanocomposite.

The photocatalytic activity of Ag₃PO₄-25 wt% TiO₂ on three pesticides (pyrimethanil, atrazine and imidacloprid) was examined (Fig. 9). The Uv-vis absorption spectrum in Fig. 9 indicated a reduction in the concentration of the pesticides after photocatalysis. Though the concentration of the three pesticides are the same, the rate of degradation differed from one pesticide to the other. The pseudo first order rate constants were calculated to be 1.319, 0.013 and 0.011 min⁻¹ for pyrimethanil, imidacloprid and atrazine, respectively. The variation in the calculated rate constants can be attributed to the different stabilities of the pesticides used for this study. The photoproducts formed from the photodegradation of pyrimethanil were identified with LC-MS.

Fig. 9 Ag₃PO₄-25 wt% TiO₂ photodegradation of (A) 10 mg L⁻¹ pyrimethanil, (B) 10 mg L⁻¹ imidacloprid, (C) 10 mg L⁻¹ atrazine; (D) calculated rate constants and removal efficiencies.

The first step in the photocatalytic degradation of pyrimethanil is the electrophilic substitution of one or two hydrogen atom in the pyrimethanil structure by hydroxyl radical.³⁵ Electrophilic substitution will form compounds 2 (m/z = 231, t_R = 1.944 min), 3 (m/z = 215, t_R = 2.034), 4 (m/z 215, t_R = 2.156 min), 5 (m/z = 213, t_R = 1.653 min), 6 (m/z = 177, t_R = 1.852 min) and 7 (m/z = 198, t_R = 3.195 min). Monohydroxylation of pyrimethanil will result in the formation of compound 4 with m/z 215 (t_R = 2.034 min). Compound 8 (t_R = 1.354 min) was formed by the hydrolysis of pyrimethanil molecule with the loss of benzene ring. Aliphatic intermediate such as compound 9 (t_R = 5.049 min) was also identified. Most of the intermediate compounds identified in this study were also identified by other researchers after photocatalytically degrading pyrimethanil.³⁶ The structure of the compounds identified by LC-MS after the photocatalytic degradation of pyrimethanil are presented in Fig. 10.

Fig. 10 The intermediate photoproducts identified by LC-MS after photodegrading pyrimethanil.

It is obvious from the present study that the Ag₃PO₄–TiO₂ composite has an enhanced photocatalytic activity when compared to pure Ag₃PO₄ and TiO₂. The photocatalytic activity of TiO₂ can be enhanced by coupling it with other semiconductors. Recently published work on Ag₃PO₄–TiO₂ has suggested that the enhanced photocatalytic activity can be attributed to the unique band matching between TiO₂ and Ag₃PO₄, inhibition of the electron hole recombination by TiO₂, and the inter semiconductor hole transfer between the valence bands of TiO₂ and Ag₃PO₄.^29a,33,37

Hydroxyl radicals, photogenerated electrons and holes, and superoxide radicals are the reactive species that are responsible for the photodegradation of organic pollutants. Benzoquinone (BQ), AgNO₃, isopropyl alcohol (IPA) and triethanolamine (TEOA) are effective scavengers for superoxide radicals (˙O₂⁻), photogenerated electrons (e⁻), hydroxyl radicals (˙OH) and photogenerated holes (h⁺), respectively.^27,30 These scavengers were therefore used to examine which of the reactive species are responsible for the degradation of rhodamine blue dye. Using Ag₃PO₄-25 wt% TiO₂, and rhodamine B solution, upon the addition of AgNO₃, IPA, TEOA and BQ, the degradation efficiency recorded reduced from 98% to 89.1%, 87.4%, 26.3% and 30.9%, respectively (Fig. 11). Therefore, photogenerated holes and superoxide radicals are the main reactive species, responsible for the photodegradation of rhodamine B dye by Ag₃PO₄-25 wt% TiO₂. This results is in agreement with the research findings of Zhang Yang et al.²⁷

Fig. 11 Effect of scavengers on the photodegradation of rhodamine B by Ag₃PO₄-25 wt% TiO₂.

A possible explanation to the trend observed in this study is schematically presented in Fig. 12.

Fig. 12 Diagram for electron (e⁻) and Hole (h⁺) separation, generation of reactive oxygen species and degradation of rhodamine B (RhB), methylene blue (MB) and pesticides [BG = Band Gap].

Ag₃PO₄ and TiO₂ can be excited by visible and UV light, resulting in the generation of photo excited electrons and holes. Under UV-visible light irradiation, the electrons in the valence band of Ag₃PO₄ are excited into the conduction band resulting in the generation of holes in the valence band of Ag₃PO₄. Since the valence band maxima position of TiO₂ (+2.7 eV vs. NHE) is relatively higher than that of Ag₃PO₄ (+2.9 eV vs. NHE),^33,37 the holes in the valence band of Ag₃PO₄ will then be transferred into the valence band of TiO₂ while the photogenerated electrons will move in the conduction band of the Ag₃PO₄. This charge transfer is possible if and only if a heterojunction is created between the TiO₂ and the Ag₃PO₄. It can be seen from the SEM images in Fig. 3 that the TiO₂ covered the surface of the Ag₃PO₄ allowing this heterojunction to be created. The photogenerated electrons and holes are therefore separated reducing the electron–hole recombination and subsequently enhancing the photocatalytic activity. The holes generated in the valence band of the TiO₂ can be used to oxidize pollutants such as dye and pesticides resulting in their degradation. In addition, the photogenerated electrons in the conduction band will react with dissolved oxygen to generate superoxide radicals. These superoxide radicals will attach pollutants and degrade them. The reactive species responsible for the degradation of organic dyes and pesticides are photogenerated holes (h⁺) and superoxide radicals (˙O₂⁻).

The photocatalytic activity of Ag₃PO₄–TiO₂ photocatalyst from this work is compared with those reported in literature. It can be seen that, the photodegradation activity of the Ag₃PO₄–TiO₂ synthesized from this work is high (Table 1).

Table 1 Comparison of the photocatalytic activity of Ag₃PO₄–TiO₂ towards the degradation of organic pollutants

Pollutant	Photocatalyst amount	Light intensity	Irradiation time	Degradation %	Ref.
Rhodamine B (200 mL, 6 mg L^−1)	100 mg	300 W tungsten halogen lamp	6 min	98%	This work
Methylene blue (200 mL, 8 mg L^−1)	100 mg	300 W tungsten halogen lamp	4 min	99.1%
Rhodamine B (50 mL, 10 mg L^−1)	50 mg	500 W Xe lamp	50 min	99.11%	38
Methyl orange (40 mL, 10 mg L^−1)	40 mg	300 W Xe lamp	60 min	95.5%	39
Methylene blue (130 mL, 20 mg L^−1)	65 mg	150 W Xe lamp	28 min	∼100%	29b
Rhodamine B (100 mL, 10 mg L^−1)	40 mg	300 W Xe lamp	10 min	∼100%	40
Bisphenol A (120 mL, 220 ug/L)	30 mg	100 W Xe lamp	10 min	∼100%	33
Methylene blue (80 mL, 10 mM)	0.03 M	500 W xenon lamp	160 min	97.1%	41
		16 W high pressure lamp	70 min	98.1%
Rhodamine B (100 mL, 0.02 mM)	50 mg	300 W Xe lamp	12 min	∼100%	29a
Rhodamine B (100 mL, 10 mg L^−1)	50 mg	500 W Xe lamp	12 min	∼100%	30
Methylene blue (75 mL, 10 mg L^−1)	40 mg	500 W high pressure lamp	12 min	∼95%	42
Methyl orange (20 mL, 20 mg L^−1)	20 mg	300 W Hg lamp	30 min	∼100%	43

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3.3 Adsorption equilibrium and kinetic studies

The adsorption equilibrium and kinetic studies were conducted using 200 mL of 5 mg L⁻¹ methylene blue solution and 100 mg of Ag₃PO₄-50 wt% TiO₂. Fig. 13 and Table 2 contain the plots and data obtained from the adsorption studies. From Fig. 13A the amount of dye adsorbed at equilibrium (Q_e) increased with increasing equilibrium concentration of the dye (C_e). To obtain a better understanding of the adsorption process, the experimental data was fitted with the Langmuir and Freundlich equilibrium isotherms. The Langmuir isotherm is given as;In this equation, C_e (mg L⁻¹) is the equilibrium concentration of the dye, Q_e (mg g⁻¹) is the amount of dye adsorbed at equilibrium, Q_m (mg g⁻¹) is the maximum adsorption capacity corresponding to a monolayer coverage and K_L (L mg⁻¹) is the Langmuir constant. The plot of Q_e vs. C_e is presented in Fig. 13A. Q_m and K_L can be estimated from the slope and intercept of linearized Langmuir isotherm expression. The estimated Q_m and K_L are presented in Table 2. To examine the favorability or otherwise of the adsorption process, an equilibrium parameter R_L was calculated from the equation below and the values presented in Table 2.K_L (L mg⁻¹) is the Langmuir constant and C_o (mg L⁻¹) is the initial dye concentration. The R_L value indicates adsorption process is irreversible when R_L is 0; favorable when R_L is between 0 and 1; linear when R_L is 1 and unfavorable when R_L is greater than 1. The maximum monolayer adsorption capacity (Q_m) and the equilibrium parameter (R_L) of the Ag₃PO₄-50 wt% TiO₂ were calculated to be 17.112 mg g⁻¹ and 0.231–0.600, respectively. These values imply that the surface of the Ag₃PO₄-50 wt% TiO₂ is homogenous and was covered with a monolayer methylene blue dye. In addition, the adsorption of methylene blue by Ag₃PO₄-50 wt% TiO₂ was favorable.

Fig. 13 (A) Langmuir and Freundlich adsorption isotherms, (B) pseudo second order (C) pseudo first order kinetics and (D) interparticle diffusion model for the adsorption of methylene blue by Ag₃PO₄-50 wt% TiO₂.

Table 2 Calculated adsorption isotherm and equilibrium kinetics parameters for the adsorption of methylene blue by Ag₃PO₄-50 wt% TiO₂

Adsorption kinetic models	Adsorption kinetic parameters
Langmuir	Qm = 17.112 (mg g^−1)
	KL = 0.3324 (L mg^−1)
	R^2 = 0.9920
	RL = 0.231–0.6 (L mg^−1)
Freundlich	KF = 4.4181 (mg g^−1 (mg L^−1)^1/n)
	n = 1.7531
	R^2 = 0.9888
Pseudo first order	Qe = 3.9886 (mg g^−1)
	K1 = 0.00341 (min^−1)
	R^2 = 0.3639
Pseudo second order	Qe = 3.5804 (mg g^−1)
	K2 = 2.2036 (g mg^−1 min^−1)
	R^2 = 0.9995
Interparticle diffusion model	KP = 2.5812 (mg g^−1 min^−1 ½)
	R^2 = 0.9189

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In addition to the Langmuir equilibrium model, the adsorption data was fitted with the Freundlich equation. The Freundlich equation is presented below;

Q_e (mg g⁻¹) is the amount of dye adsorbed on Ag₃PO₄-50 wt% TiO₂ at equilibrium, C_e is the equilibrium concentration of the dye in solution, K_F (mg g⁻¹(mg L⁻¹)^1/n) and 1/n are the Freundlich constants related to the adsorption capacity and the adsorption intensity, respectively. Higher values of K_F indicate higher affinity for adsorption and the values of 1/n lie between 0.1 < 1/n < 1, indicating favorable adsorption.⁴⁴ The Freundlich constants were calculated from the linearized Freundlich isotherm expression and presented in Table 2. The value of 1/n is less than 1 indicating favorable adsorption process. The K_F and a correlation coefficient were estimated as 4.4181 mg g⁻¹ (mg L⁻¹)^1/n and 0.9888, respectively. Since the correlation coefficient for both the Langmuir and Freundlich model were greater than 0.95, the adsorption process may have followed both models. It is however likely the adsorption process followed the Langmuir isotherm model since it had a relatively higher correlation coefficient when compared to Freundlich isotherm.

The mechanism and rate controlling step of the entire adsorption process was examined using the following adsorption kinetic models; pseudo-first order, pseudo-second order and intra-particle diffusion models. The pseudo-first order equation is presented below;

Q_e and Q_t (mg g⁻¹) are the amount of the dye adsorbed at equilibrium and at time t (min), respectively. K₁ (min⁻¹) is the pseudo-first order rate constant. The parameters K₁ and Q_e were estimated from Fig. 13C and presented in Table 2. The correlation coefficient obtained is low (R² = 0.3639). This suggests that the pseudo-first order kinetic model is not suitable to describe the adsorption process.

The adsorption kinetics was also investigated using pseudo-second order kinetic model presented below;

K₂ (g mg⁻¹ min⁻¹) is the pseudo-second order rate constant. Q_e and Q_t (mg g⁻¹) are the amount of dye adsorbed at equilibrium and at time t (min), respectively. Q_e and K₂ were calculated from Fig. 13B. The high correlation coefficient of 0.9995 suggests that the adsorption process follows a pseudo-second order adsorption kinetics process.

The Langmuir adsorption isotherm confirm the monolayer adsorption of methylene blue onto the surface of the Ag₃PO₄-50 wt% TiO₂. The dye molecules were probably transported from the dye solution to the Ag₃PO₄-50 wt% TiO₂. The possibility of intra-particle diffusion was therefore examined using the intra-particle diffusion model⁴⁵ presented below;

Q_t = K_pt^1/2 + C (6)

Q_t (mg g⁻¹) is the amount of methylene blue dye adsorbed at time t (min), C is the intercept and K_p (mg g⁻¹ min⁻¹) is the intra-particle diffusion rate constant. K_p was estimated from Fig. 13D and presented in Table 2. The plot in Fig. 13D shows a double straight line. The initial stage represents the transportation of methylene blue to the external surface of the Ag₃PO₄-50 wt% TiO₂ through film diffusion within a very short time. The first linear part can be due to the entry of methylene blue molecules into the Ag₃PO₄-50 wt% TiO₂ by intra-particle diffusion.⁴⁶ The final equilibrium stage is represented by the second linear portion of Fig. 13D.

The results presented in this article revealed that TiO₂–Ag₃PO₄ has the potential of removing water pollutants through adsorption and photocatalysis. TiO₂–Ag₃PO₄ therefore has a potential application in the development of water treatment technologies.

4 Conclusion

In this study, TiO₂ semiconductor was coupled with Ag₃PO₄. SEM images of the as-prepared TiO₂–Ag₃PO₄ showed a covering of the Ag₃PO₄ surface by TiO₂ creating a heterojunction between coupled semiconductors. DRS analysis and optical band gap estimation revealed two absorption thresholds corresponding to TiO₂ and Ag₃PO₄. TiO₂–Ag₃PO₄ showed strong photocatalytic activity for rhodamine B, methylene blue, pyrimethanil, imidacloprid and atrazine. LC-MS analysis of the photodegraded pyrimethanil resulted in the identification of hydroxylated and aliphatic derivatives of pyrimethanil. The photocatalytic activity was dependent on the weight percent of the TiO₂ used in the synthesis with 25 wt% TiO₂ recording the highest photocatalytic removal efficiency and rate constant. The synergy between the TiO₂ and Ag₃PO₄ semiconductors was attributed to the good band gap matching that resulted in the efficient separation of charges which in turn led to a suppression of the electron–hole recombination. The TiO₂–Ag₃PO₄ showed strong adsorption towards methylene blue dye and the adsorption equilibrium isotherm and kinetics followed the Freundlich and Langmuir isotherms, and pseudo second order rate kinetics, respectively. TiO₂–Ag₃PO₄ therefore removes the pollutants by both photocatalytic and adsorption process. The results from this study showed the potential application of TiO₂–Ag₃PO₄ in water treatment technologies.

Conflicts of interest

There is no conflict to declare.

Acknowledgements

This work was financially supported through Commonwealth Academic Fellowship funded by the UK Government.

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