Physical and chemical properties of the prepared catalystsFTIR analysis was used to identify the characteristic functional groups of the obtained samples. In Fig. 2a, Ag3PO4 showed a sharp beak at 884.67 cm−1 which is assigned to the vibration mode of (PO4–3) group. The small peak at 1384.88 cm−1 confirms the presence of residual water molecules because of the (OH−) vibration bond. The stretching modes at 3464.65 cm−1 and 1634.74 cm−1 are caused by the OH– defects23,33. FTIR analysis of Ag3PO4/G/SiO2 nanocomposite manifested a bond of C–Si at 2114.32 cm−1, and bending vibration of OH− at 1642.58 cm−1. The absorption band at 1280.13 cm−1 is corresponding to O–Si–O bond23,33,34.Figure 2(a) FTIR and (b) XRD of Ag3PO4 and Ag3PO4/G/SiO2 composite.Figure 2b shows the XRD patterns of the of Ag3PO4 and Ag3PO4/graphene/SiO2 which revealed a high crystallinity of both samples. The diffraction peaks at the 2θ values 30.500 and 31.680 corresponding to the reflections of (1 2 0) and (− 2 1 2) represent the Ag3PO4 monoclinic phase structure in both catalysts and agreed with (PDF 72–0122). For the composite a sharp peak observed at plane (2 2 1) which incident for nano silica crystals (PDF 85–0621), and the lattice plane (1 0 0) reveals formation of graphene sheets34.The formed peak at 2θ of 28.980 indicates the formation of monoclinic phase of silver silicate with (0 2 4) plane (PDF 85–0281), which stresses the attachment of Ag3PO4 particles with graphene/SiO2 composite through the nucleation on the PVP polymer chain33. All observed XRD miller indices were indexed in Table 1. The morphological structure of Ag3PO4 nanoparticles and Ag3PO4/G/SiO2 nanocomposite were investigated by SEM micrographs and the results are shown in Fig. 3a,b. It can be observed that the particles of Ag3PO4 consist of irregular agglomerated rods and spheres. Moreover, the images showed a lack of symmetry and uniformity in distribution and shape as presented in Fig. 3a35. The micrographs of Ag3PO4/G/SiO2 exhibited small particles of silica attached to the graphene surface with spheres and rods of Ag3PO4 (Fig. 3b). TEM images of the prepared samples showed that the rods and spheres of the silver phosphate are formed in random distribution (Fig. 3c). Whereas, in Fig. 3d, the images showed a semitransparent layer of graphene attached on its surface nano-spheres of silica distributed in random arrangement and rods and spheres of silver phosphate33,34,35. Figure 3e,f show SAED pattern of prepared Ag3PO4 and Ag3PO4/G/SiO2 composite in which the bright spots correspond to the cubic Ag3PO4 phase were appeared and confirm the well crystallinity of both catalysts.Table 1 Miller indices (h k l) of observed XRD peaks.Figure 3(a and b) SEM of Ag3PO4 and Ag3PO4/G/SiO2 composite. (c and d) TEM of Ag3PO4 and Ag3PO4/G/SiO2 composite. (e and f) SAED patterns of Ag3PO4 and Ag3PO4/G/SiO2 composite.Figure 4 shows the variation of EDS elemental analysis for the photocatalysts compositions. For Ag3PO4 (Fig. 4a), EDS analysis shows that Ag percentage is about 82%, P percentage of 10% and 5% of oxygen atoms. While for Ag3PO4/G/SiO2 composite (Fig. 4b) the percentages of silver and phosphorous will decreases to 62 and 3% respectively, after compositing with graphene/silica. Otherwise, the oxygen percentage increases to about 17% and the carbon percentage was nearly to 8% with a 2% silicon. These analyses are compatible with XRD analysis.Figure 4EDS elemental analysis of Ag3PO4 (a) and Ag3PO4/G/SiO2 composite (b).Specific surface area of the as-synthesized photocatalysts was estimated by N2 adsorption technique and the results analyzed using BET theory. BJH analyses was performed to measure the average pore diameter of both catalysts and showed in Fig. 5a,b, where the average pore diameter of the composite was 5.4 nm, while for Ag3PO4 was 44.28 nm. Figure 5c showed that the Ag3PO4/G/SiO2 nanocomposite has large specific surface area which about 84.97 m2/g compared with 1.53 m2/g for Ag3PO4, the increase in specific area may be due to the incorporation of the highly surface area graphene/SiO2 composite which was previously prepared by Amr et al.33. Table 2 summarize the BET surface area analysis, it is observed that the Ag3PO4/G/SiO2 nanocomposite has smaller pores and also has a larger specific surface area per one gram, which indicates the presence of a large number of pores, i.e., its porosity34, which means the nanocomposite has greater ability for adsorption as mentioned before. Figure 5(a) BJH plot of Ag3PO4, (b) BJH plot of Ag3PO4/G/SiO2 (c) N2 Adsorption–Desorption isotherm on the surface of Ag3PO4 and Ag3PO4/G/SiO2.Table 2 Summarization of BET analysis of Ag3PO4 and Ag3PO4/G/SiO2.In order to study the optical behavior of Ag3PO4 and Ag3PO4/G/SiO2, UV/VIS DRS have been performed. As reported before, Ag3PO4 nanoparticles have a great sensitivity to visible light. The results showed that both as-prepared catalysts can absorb visible light, however, the nanocomposite revealed higher absorption capability as showed in Fig. 6a. This can be explained via the high conductivity of graphene which could act as an electron acceptor which in role enhance the rate of formation of positive holes on the composite surface and ensure the continuity of the photocatalytic performance25,33,36. Tauc equation25,36 has been used to determine the band gap energy of Ag3PO4 and Ag3PO4/G/SiO2 sample, according to the following formula.$$ \alpha h\nu = {\rm A}\left( {h{\varvec{\nu}} – {\text{E}}_{g} } \right) ^{{{\raise0.7ex\hbox{$n$} \!\mathord{\left/ {\vphantom {n 2}}\right.\kern-0pt} \!\lower0.7ex\hbox{$2$}}}} $$
(6)
where Eg, α, h, \({\varvec{\nu}}\), and A, are band gap energy, absorption coefficient, Planck constant, photon frequency, constant, respectively. Figure 6b showed the plot of \((\alpha {\text{h}}\nu )^{2}\) (indirect transition) versus energy band (\({\text{h}}\nu\))35. According to the plot the band gap energy of Ag3PO4 was 2.4 eV and for the composite was 2.307 eV; hence, the obtained results enhance the strongest photocatalytic property of the composite.Figure 6UV–VIS spectra of the Ag3PO4 and Ag3PO4/G/SiO2 (a), Tauc relationships of the prepared samples (b).Studying the effect of operational parameters on the photocatalytic degradation efficiencyIn order to study photocatalytic activity at different parameters, all the experiments were conducted first in dark medium for 30 min before exposure to the visible light.pH effectThe pH effect on photodegradation rate is shown in Fig. 7a, indicating the great effectiveness of both catalysts in the acidic medium, where the surfaces of the as-synthesized catalysts carry positive charges, which is consistent with the pHzpc investigation. According to the graph, at pH = 2, the percentage of dye degradation employing Ag3PO4 and Ag3PO4/G/SiO2 reached up to 89% and 98.5%, respectively. After that, they showed a gradual degradation decrease due to a decrease in the number of positive charges on catalyst surfaces. After the neutral medium, the degradation percent sharply decreased to approximately 10%, because the catalyst surfaces had become negatively charged according to pHzpc (Figure S1), resulting in generating an electrostatic repulsion force25,34.Figure 7(a) pH, (b) catalyst dose, (c) and (d) time and initial concentration effects on the rate of degradation of TB using Ag3PO4 and Ag3PO4/G/SiO2.Effect of catalyst doseThe effect of the catalyst dose was investigated to determine its effect on accelerating the degradation efficiency of the TB dye. As shown in Fig. 7b, as the catalyst dose increases, the percentage of TB degradation increases. This could be illustrated as by increasing the dose, sufficient surface area became available to serve the adsorption process, besides more radicals and positive holes will be generated which in turn serve the degradation rate34,35. The composite had a higher degradation efficiency than Ag3PO4, which was in line with BET analysis expectations because the composite had a higher specific surface area than Ag3PO4; thanks to the incorporated graphene/SiO2 composite which increased the adsorption area and improved the photocatalytic activity by trapping the excited electrons in the conduction band, therefore, minimizing their recombination with the valence band’s holes, which had a positive impact on the percent of degradation36,37. It is demonstrated from the graph that Ag3PO4 and Ag3PO4/G/SiO2 have a significant percentage of degradation at dose 0.03 g, with 89% and 98.5%, respectively. When the dose was increased up to 0.05 g the degradation percent was slightly increased. As a consequence, the appropriate used dose of each photocatalyst was 0.03 g.Initial dye concentration and contact time effectsThe effect of the initial dye concentration on the efficiency of photodegradation of TB dye was conducted at different initial concentrations (20, 30, 40, 50, 100 ppm) at constant operating conditions as shown in Fig. 7c,d. The results showed that as the initial concentration of the dye increases, the removal efficiency decreases34. For instance, for initial dye concentration 20 ppm, pH 2, and catalyst dosage 0.03 g the percent of degradation reached up to 89%, and 98.7% for Ag3PO4, and Ag3PO4/G/SiO2, respectively. As a consequence, the study emphasis the boosting effect of the composite in the degradation of TB dyes. Furthermore, the dye’s degradation tendency revealed a rapid response in the first two minutes, but after careful analysis of the results, it appears that the composite is more active than pure Ag3PO4 in terms of degradation rate, followed by a slight degradation increase over the next four minutes, and finally a steady-state response. On the other hand, Ag3PO4 performed well in the first two minutes, but with a smaller percentage of degradation; after that, a progressive increase was detected for the next six minutes; eventually, the graph revealed a constant state for the final five minutes. The behavior of the photocatalysts is agreed with the BET results.Surface adsorption isothermIn order to identify and study the behavior of the interaction between the dye and the adsorbent. Three adsorption isotherm models, which are Langmuir, Freundlich and Temkin, have been carried out. By analyzing the adsorption experimental data in dark medium, the most appropriate model will be utilized to investigate the kinetic model that the adsorption process follows34,38,39,40,41.Langmuir isothermThe data was studied and analyzed using the Langmuir model, based on the linear formula of Langmuir Eq. (7). The separation factor (RL), a dimensionless constant, was also determined in order to express the isotherm’s crucial properties using the following equation formula (8):$$ \frac{{C_{e} }}{{q_{e} }} = \frac{1}{{K_{L} q_{max} }} + \frac{1}{{q_{max} }} $$
(7)
$$ R_{L} = \frac{1}{{1 + C_{i} K_{L} }} $$
(8)
where Ci and Ce are the adsorbate initial and equilibrium concentration (mg/l), respectively. qe and qmax are the adsorption capacity adsorbed at equilibrium, and the maximum capacity (mg/g) respectively, and KL is the Langmuir adsorption constant (l/mg). A plot of Ce/qe versus Ce was produced to determine qmax and KL as shown in Fig. 8a and Table 3. When the RL value is equal to zero, the adsorption is irreversible; when the value is between zero and one, the adsorption is favorable; when the value is larger than one. the adsorption is unfavorable; and when the value is equal to one, the adsorption is linear42,43,44. The calculated RL values are shown in Table 3. All the values were dropped between zero and one which is favorable.Figure 8Adsorption isotherm models of TB on Ag3PO4 and Ag3PO4/G/SiO2. (a) Langmuir isotherm, (b) Freundlich isotherm and (c) Temkin isotherm.Table 3 The values of the separation factor RL at different initial dye concentration using Ag3PO4, and Ag3PO4/G/SiO2.Freundlich isothermThe model that is built on heterogeneous adsorbent surfaces, was also used to experience the results. This isothermal model accomplished its hypothesis as a term that illustrates the heterogeneity of the surface and the exponential distribution of available sites. The linear formula of the isotherm is figured out in Eq. (9),44.$$ Log q_{e} = Log K_{f} + \frac{1}{n} Log C_{e} $$
(9)
where Kf is adsorption capacity (L/mg), 1/n is adsorption intensity, Ce is the adsorbate equilibrium concentration (mg/L), qe is the adsorption capacity adsorbed at equilibrium (mg/g). Figure 8b depicts a plot that expresses the relation between log qe and log Ce. A linear graph of Log (qe) against Log (Ce) was drawn to detect the values of the slope (1/n) and intercept (Log Kf) as shown in Fig. 8b and Table 4.Table 4 The equilibrium parameter for the pre-mentioned three isothermal models.Temkin isothermThis model assumes that as the concentration of adsorbate on the adsorbent surface increases, the heat of adsorption of all molecules in the layer decreases linearly due to interactions between them, and that the isothermal model is distinguished by symmetric distribution of binding energies up to maximum binding energy. Equation (10) presents the linear formula of Temkin isotherm.$$ q_{e} = \frac{R}{{B_{T} }} ln K_{T} + \left( {\frac{R}{{B_{T} }}} \right)\ln C_{e} $$
(10)
where qe is the amount adsorbed at equilibrium (mg/g), Ce is the adsorbate equilibrium concentration (mg/L), R is gas constant, KT (L/mg) and BT are constants determined by graphing qe against ln Ce, as shown in Fig. 8c. Constants can be easily identified using the intercept and slope as shown in Fig. 8c and Table 437,38,39,40.The correlation coefficient of (R2) identifies the best appropriate model regarding the different equilibrium parameters for Langmuir, Freundlich, and Temkin isotherms for TB adsorption using Ag3PO4 and Ag3PO4 / G /SiO2.From the previous data, qmax for the new composite is higher than that was calculated for graphene/SiO2 as it was 376 mg/g34, which ensure the greater adsorption capacity of Ag3PO4/graphene /SiO2. It can be concluded that the catalysts are more likely to follow the Langmuir isotherm model than other models which also revealed a well-fitting for the data. To sum up, the adsorption kinetic models fit the isothermal models in the following arrangement Langmuir > Temkin > Freundlich.photodegradation kinetic modelingIn order to study and investigate the mechanism of the photodegradation process, kinetic models have been employed. To explore the kinetics of the degradation process, pseudo first order and pseudo second order models have been constructed. Figure 9 shows the first and the second order models for TB elimination using Ag3PO4 and Ag3PO4/G/SiO2. A comparison of the experimental data was conducted in order to identify the model that was most appropriate for the obtained results. The optimal model was identified based on the degree of agreement between the computed and experimental values (qe), as well as the correlation coefficient R245,46,47,48. Equations (11) and (12) show the linearized formulas for pseudo first and second order models, respectively.$$ \ln \left( {q_{e} – q_{t} } \right) = lnq_{e} – K_{1} t $$
(11)
$$ \frac{t}{{q_{t} }} = \frac{1}{{K_{2} q_{e}^{2} }} + \frac{1}{{q_{e} }} $$
(12)
Figure 9First order plots for various initial concentrations of TB removal using Ag3PO4 (a), Ag3PO4/G/SiO2 (b) and second order plots for various initial concentrations of TB removal using Ag3PO4 (c), Ag3PO4/G/SiO2 (d).The experimental results for TB removal using Ag3PO4 and Ag3PO4/G/SiO2 are displayed in Tables 5 and 6, and proved that the photodegradation process fit more the second order kinetic model as the resultant experimental capacities were approximately equals to the calculated one. Moreover, the correlation factors R2 were also equal to 0.99 or higher which emphasis the convenience of the second-order.Table 5 The pseudo first and second order kinetic parameters for TB removal using Ag3PO4.Table 6 The pseudo first and second order kinetic parameters for TB removal using Ag3PO4/G/SiO2.From the previous data shown in Tables 3, and 4, it could be concluded that the nanocomposite showed higher reaction rate constants (K2). The average of the rate constants increased by 2.53 times that of Ag3PO4. This is can be explained as the composite possess a larger surface area and better-regulated morphology with well particles distribution; therefore, the degradation efficiency is promoted. BET, SEM, and TEM analyses corroborate these findings26.Proposed photodegradation mechanismWhen the silver phosphate is irradiated with visible light, electrons in the material are excited to a higher energy level, creating electron–hole pairs. The electron–hole pairs thus created are separated due to the presence of G/SiO2 nanocomposite, which traps the photo-excited electrons. The excited electrons can reduce the atmospheric oxygen to O2. radical which could lead to the breakdown of pollutants to carbon dioxide and water, and the holes present in the valance band will react with water molecules and generate hydroxyl radicals (OH.) that are very strong oxidizing agents, which helps in the degradation of TB dye41,42. Also, the adsorption of oxygen onto the G/SiO2 surface will continuously oxidize the TB dye and release carbon dioxide and water. The regenerated electrons can then recombine with the holes trapped in the bulk of the Ag3PO4/G/SiO2 nanocomposites to produce more excited electron–hole pairs, which can repeat the photocatalytic process as shown in Fig. 1. The scavenger experiments were conducted to determine the key active species (h+, e, ·OH and ·O2) involved during the photocatalytic process. As seen in Figure S5, the significant reduction was observed on addition of propanol, suggesting that ·OH was the dominant species during the degradation of dyes. The addition of oxalic acid (h+ and methanol (·O2) scavengers resulted in only slight reduction of photo- activity. The order of photocatalytic suppression after addition of different scavengers were ·OH > e− > h+>·O2.In brief, the Ag3PO4/G/SiO2 composites act as a visible light photocatalyst for the degradation of organic pollutants, and G/SiO2 playing a major role by acting as a support to promote dye adsorption, an electron acceptor and also delaying the recombination rate of electron–hole pairs, which are responsible for the photocatalytic reactions43.Characterization of the used catalystSome characteristic analyses were performed to the used photocatalysts in order to inspect their morphology and status, seeking to proof that, all the adsorbed dye molecules on the catalyst surface were degraded after light illumination which in turn boost the efficiency of both photocatalysts. FTIR, EDS, and SEM–EDS mapping tests are shown below. Where Fig. S2, FTIR analysis of the used catalysts after they were centrifuged at 6000 rpm and dried for 12 h in a vacuum oven at 60 °C to ensure the degradation of TB dye and the absence of adsorbed molecules on the catalyst surface. It was predicted that the FTIR result would reveal the same functional groups for each catalyst as mentioned before. SEM–EDS mapping results are shown in Fig. S3 and Fig. S4, from these results, we can conduct the coexistence of both catalysts’ elements and no elements of the dye were present which is compatible with the FTIR results. These results ensure the degradation of the TB dye.A comprehensive study has been done to investigate and compare the synthesized catalysts’ activity and efficiency in comparison with other photocatalysis used for the degradation of trypan blue as an organic pollutant. Table 7 shows the comparison with respect to TB as a pollutant source.Table 7 A comparison of photocatalytic degradation efficiency using various catalysts, in terms of TB degradation.
Formulation of silver phosphate/graphene/silica nanocomposite for enhancing the photocatalytic degradation of trypan blue dye in aqueous solution
