CharacterizationThe XRD patterns of the synthesized CoFe2O4 nanoparticles, ferrisilicate and CoFe2O4 /ferrisilicate nanocomposites are depicted in Fig. 1. The pattern of CoFe2O4 nanoparticles (Fig. 1a) represents diffraction peaks at 18.55°, 30.55°, 35.97°, 37.62°, 43.77°, 54.37°, 57.79°, and 63.44°, which are consistent with the standard pattern of CoFe2O4 with cubic crystal structure (JCPDS Card No. 22–1086). The peaks are attributed to the reflections from planes with the Miller indices of (111), (220), (311), (222), (400), (422), (511) and (440), respectively. Based on the FWHM (Full-width at half maxima) values of three main peaks ((220), (311) and (400)), the mean size of CoFe2O4 crystallites was calculated to be 47nm by using the Scherrer’s equation (\({\text{D = }}\frac{{{\text{K}}\uplambda }}{{\upbeta {\text{Cos}}\uptheta }}\)).Fig. 1XRD patterns of (a) CoFe2O4, (b) ferrisilicate zeolite, (c) CF10%/FS, (d) CF25%/FS and (e) CF50%/FS.The formation of the ZSM-5 type ferrisilicate zeolite is proven by the presence of peaks at 7.97°, 8.84°, 23.08°, and 23.9°, which are ascribed to the (011), (020), (051) and (033) planes, respectively. According to the patterns of the CoFe2O4 /ferrisilicate nanocomposites, the peaks related to CoFe2O4 have appeared along with the peaks of ferrisilicate and intensified by increasing the percentage of the CoFe2O4 in the composites.Figure 2 demonstrates the FT-TR spectra of the synthesized samples. The fundamental vibrations of the ions in the crystal lattice of metal ferrites occur in the range of 300–700 cm−137.The appearance of two main absorption bands in this region is a common feature of these compounds37. Here, for CoFe2O4 nanoparticles, two peaks at 420 cm−1 and 582 cm−1 are assigned to the stretching vibrations of metal–oxygen bonds with octahedral and tetrahedral coordination, respectively38. Moreover, in the ferrisilicate sample, the peaks attributed to the bending vibrations in TO4 tetrahedra, external T–O–T symmetric and internal O–T–O asymmetric stretching vibrations can be observed at 450 cm−1, 800 cm−1 and 1095 cm−1, respectively36. In addition, the characteristic peaks of ZSM-5 type zeolite at 549 cm−1 (double ring vibrations) and 1222 cm−1 (external T–O–T asymmetric stretching vibration) confirm the zeolitic structure of the ferrisilicate39. The mentioned peaks are present in the spectra of the CoFe2O4/ferrisilicate composites, as well. A more detailed look on these spectra indicates a broadening of the peak at 549 cm−1 occurs and the absorption in this region increases from CF10%/FS sample to CF50%/FS. It can be imputed to the presence of a strong absorption band of CoFe2O4 at 582 cm−1 and its overlap with the peak of zeolite at 549 cm−1. Therefore, the relative intensity of the peaks of CoFe2O4 to zeolite increases by increasing the weight percentage of the cobalt ferrite in the composites.Fig. 2FTIR spectra of (a) CoFe2O4, (b) ferrisilicate zeolite, (c) CF10%/FS, (d) CF25%/FS and (e) CF50%/FS.The morphology and elemental composition of the samples were investigated using scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS). The bare CoFe2O4 particles are agglomerated owing to their magnetic characteristics, as seen in Fig. 3a, b. The presence of Fe and Co in this sample is proven from the EDS spectrum. On the other hand, Fig. 3c demonstrates the ferrisilicate zeolite comprised microspheres with rugged surfaces. These spheres are composed of merged cuboids containing smaller nanoparticles (Fig. 3d).Fig. 3FE-SEM images of (a, b) CoFe2O4 and (c, d) ferrisilicate zeolite.Through the synthesis of CoFe2O4/ferrisilicate composites, the surfaces of the zeolite particles are found to be covered by the cobalt ferrite nanoparticles with the sizes lower than 50 nm (Fig. 4). Hence, the particle size of ferrite can be significantly reduced by the synthesis of CoFe2O4 on ferrisilicate. Here again, the magnetic feature of the CoFe2O4 leads to the aggregation of the zeolite particles. The EDS spectra confirm the incremental loading amount of the CoFe2O4 on the ferrisilicate by increasing its weight percentage in the samples. The enhancement of relative intensities of Co and Fe peaks are observable from the spectra (Fig. 5). Additionally, the loaded nanoparticles with the sizes below 50 nm on the ferrisilicate microspheres are clearly depicted in TEM images of the CF50%/FS sample (Fig. 6).Fig. 4FE-SEM images of (a–c) CF10%/FS, (d–f) CF25%/FS and (g–i) CF50%/FS.Fig. 5EDS spectra of (a) CoFe2O4, (b) ferrisilicate zeolite, (c) CF10%/FS, (d) CF25%/FS and (e) CF50%/FS.Fig. 6TEM images of CF50%/FS nanocomposite.Thermogravimetric analysis was performed on the uncalcined cobalt ferrite nanoparticles. The TGA–DTG curves of the sample are exhibited in Fig. 7. As is evident, a weight loss occurs in the region of 20–140 °C, which is arising from the egress of moisture and water molecules from the structure. In addition, a substantial step of the weight reduction process has appeared in the range of 180–320 °C. An intense weight loss with a sharp peak at ~ 200 °C in the DTG curve can be associated with the decomposition and oxidation of citrate. Moreover, two shoulders at 240 °C and 280 °C can imply the combustion of nitrate ions as well as hydroxyl and carboxylate groups coordinated the metal centers. The absence of considerable weight loss at the temperatures higher than 320 °C reveals the completely removal of excess compounds except the metal oxide phase. The total weight loss of the sample up to 800 °C is approximately 79.15%.Fig. 7TGA and DTG curves of the CoFe2O4 before calcination.Diffuse reflectance spectroscopy was used to evaluate the optical properties of the photocatalysts and determine their band gaps. Figure 8a demonstrates the absorption spectra of CoFe2O4 and CF25%/FS samples. Both have wide absorption in the visible region up to wavelength of 800 nm. Therefor they can be used as photocatalysts under visible light irradiation. To calculate the band gap energies, Kubelka–Munk function (\(F\left(R\right)= {(1-R)}^{2}/2R\) , where R is reflectance) was used. From the Tauc plots presented in Fig. 8b, both samples have the band gaps of 1.57 eV.Fig. 8(a) UV–vis absorption spectra and (b) Tauc plots of CoFe2O4 and CF25%/FS.Nitrogen adsorption/desorption isotherms of CoFe2O4, CF10%/FS, CF25%/FS, and CF50%/FS were provided for the exploration of the surface areas and pore size distribution. As illustrated in Fig. 9a–d, the isotherms of CF10%/FS, CF25%/FS, and CF50%/FS represent a combination of type I and IV isotherms, while the recorded profile for ferrite is consistent with the type III isotherm. For all samples, no plateau was observed at high pressure, corroborating the presence of abundant mesopores (pore size between 2 and 50 nm) and macropores (pore size larger than 50 nm) in the resultant CF10%/FS, CF25%/FS, and CF50%/FS nanocomposites. Based on the BET model, the surface areas for CoFe2O4, CF10%/FS, CF25%/FS, and CF50%/FS are estimated to be 2.4, 301.7, 241.8, and 139.7 m2 g−1, respectively. As can be seen, by increasing the content of ferrite nanoparticles in composites, the BET surface area decreases due to the blocking of zeolite pores. Additionally, the BJH curves (Fig. 9e-g) were utilized to observe the pores size distribution in the samples. As illustrated, two types of pore (mesopores and micropores (2˃ pore size)) are illustrated in the BJH profiles relevant to the nanocomposites. On the other hand, loading higher amounts of nanoparticles leads to the larger pores, so that CF50%/FS is predominantly mesoporous.Fig. 9Nitrogen adsorption/desorption isotherms of (a) CoFe2O4, (b) CF10%/FS, (c) CF25%/FS and (d) CF50%/FS. The BJH curves for (e) CF10%/FS, (f) CF25%/FS and (g) CF50%/FS.Vibrating sample magnetometer (VSM) analysis was employed to assess the magnetic characteristics of the samples. In Fig. 10, the M–H curves of CoFe2O4, CF25%/FS and CF50%/FS demonstrate hysteresis loops that imply the ferromagnetic behavior40. The three key parameters including saturation magnetization (Ms), remanent magnetization (Mr), and coercivity (Hc) are presented in Table 1. It is likely that the low Ms value in CF25%/FS is due to the low proportion of cobalt ferrite in the sample. The squareness ratio (Rs = Mr/Ms) for the samples was evaluated and the results are presented in Table 1. The obtained values, which are lower than 0.5, indicate that all the samples possess multi-domain structures. In addition, the small coercivity in all samples suggests their soft magnetic character40. Nevertheless, the coercivity of the composites is greater than that of the bare CoFe2O4, which can be attributed to the reduced particle size of cobalt ferrite within the composites. It was reported that in the multi-domain particles, the coercivity rises as the particle size decreases, reaching a maximum level at the critical diameter associated with the transition from multi to single-domain state41.Fig. 10M–H curves of the (a) CoFe2O4, (b) CF25%/FS and (c) CF50%/FS.Table 1 Ms, Hc, Mr and Rs parameters of CoFe2O4, CF25%/FS and CF50%/FS.Photocatalytic production of ammoniaThe performances of the synthesized samples in the photocatalytic reduction of N2 to ammonia were evaluated under visible light irradiation. As previously mentioned, the concentration of the ammonia was quantified via UV–vis spectroscopy following the addition of Nessler’s reagent. As a result of the reaction between ammonia and Nessler’s solution, OHg2NH2I is formed according to the reaction (1), which leads to the yellowing of the solution42:$${2}\left( {{\text{HgI}}_{{2}} .{\text{2KI}}} \right) \, + {\text{ NH}}_{{4}} {\text{OH }} + {\text{ 3NaOH}} \to {\text{OHg}}_{{2}} {\text{NH}}_{{2}} {\text{I }} + {\text{ 3H}}_{{2}} {\text{O }} + {\text{ 4KI}} + {\text{ 3NaI}}$$
(1)
The obtained results for the synthesized samples are presented in Fig. 11. As is apparent from the figure, in the case of bare CoFe2O4, the concentration of the ammonia increases over time and reaches the maximum point (755.53 µmol L−1) after 120 min light irradiation. A decline in the concentration is observable with further irradiation time, which can be attributed to the photodegradation of ammonia43,44. To scrutinize the mechanism of the nitrogen photoreduction over CoFe2O4 nanoparticles, the band gap energy and band edge positions should be assessed.Fig. 11The performances of the synthesized samples in photocatalytic ammonia production.The synthesized CoFe2O4 with the band gap of 1.57 eV can absorb the light in the visible region and generate electrons and holes. The energy level of the CB in CoFe2O4 was determined to be − 0.4 V in our previous work33. By using the formula of EVB = ECB + Eg, the energy of the VB is calculated as 1.17 V. The energy of the VB (1.17 V vs NHE, pH = 7) is more positive than the water oxidation potential (O2/ H2O, 0.81 V vs NHE)13 and the CB level (− 0.4 V vs NHE) is more negative than the nitrogen reduction potential (N2/NH3, − 0.278 V vs NHE)44. Therefore, by providing electrons and protons the conversion of nitrogen to ammonia can take place as belows13:$${\text{H}}_{{2}} {\text{O }} + {\text{ 2h}}^{ + } \to {\text{2H}}^{ + } + \frac{1}{2}{\text{O}}_{{2}}$$
(2)
$${\text{N}}_{{2}} + {\text{ 6H}}^{ + } + {\text{ 6e}}^{ – } \to {\text{2NH}}_{{3}}$$
(3)
In addition to the proper potential energies, the free charge carriers are required for proceeding a photocatalytic process. In this respect, one of the methods to prevent e−-h+ recombination is utilizing scavengers45. Here, methanol as a hole-scavenger has boosted the ammonia production. In terms of the mechanism of methanol action, there are different scenarios that can be considered46: (1) According to the (Eq. 4) methanol can directly consume the holes in VB and prevent their recombination with the electrons in CB or (2) by following the reaction (5) it can react with the ºOH which can oxidize the produced NH3 and decrease its concentration.$${\text{CH}}_{{3}} {\text{OH }} + {\text{ h}}^{ + } \to {}^{ \circ }{\text{CH}}_{{2}} {\text{OH }} + {\text{ H}}^{ + }$$
(4)
$${\text{CH}}_{{3}} {\text{OH }} + {}^{ \circ }{\text{OH}} \to {}^{ \circ }{\text{CH}}_{{2}} {\text{OH }} + {\text{ H}}_{{2}} {\text{O}}$$
(5)
To carry out the second reaction, the possibility of the presence of hydroxyl radicals in the solution should be investigated. These radicals can be produced through the reactions below47:$${\text{H}}_{{2}} {\text{O}} \to {\text{OH}}^{ – } + {\text{ H}}^{ + }$$
(6)
$${\text{OH}}^{ – } + {\text{ h}}^{ + } \to {}^{ \circ }{\text{OH}}$$
(7)
Theoritically, the redox potential of OH−/ºOH (1.99 V vs NHE)44 is more positive than the potential of the VB of CoFe2O4, thereby no ºOH can be generated. For a practical approach to investigating the presence of ºOH in the reaction system, terephthalic acid was introduced as ºOH scavenger agent. Referring to Fig. 12, it can be noticed that TA addition did not influence the ammonia concentration, suggesting that ºOH may not exist in the photocatalytic system and the second mechanism (Eq. 5) is improbable. On the other hand, the substituting TA with methanol increases the ammonia production. Based on these observations, it can be inferred that methanol impedes the electron–hole recombination by prompt grabbing of holes from VB and thus promote the ammonia production through the first mechanism (Eq. 4). It should be noted that by comparing the energy level of VB and redox potential of NO3-/NH3 (+ 0.363 vs NHE)44, the oxidation of ammonia as a side reaction can thermodynamically happen over the synthesized photocatalyst and decrease the concentration of NH3.Fig. 12Trapping test of ºOH and h+ in photocatalytic reaction using CF50%/FS sample.By loading various amounts of CoFe2O4 (10%, 25% and 50%) on the ferrisilicate zeolite in CoFe2O4/ferrisilicate composites, the maximum ammonia concentration is changed to 599.11, 896.18 and 969.48 µmol L−1, respectively. It is clear that comparing to the bare CoFe2O4, the ammonia production is decreased for CF10%/FS owing to the lowest amount of cobalt ferrite photocatalyst in this composite. On the other hand, for the two other composites, the ammonia concentration is elevated and it reaches to the highest amount in the CF50%/FS sample. Large BET surface areas of 301.7, 241.8, and 139.7 m2 g−1 in the synthesized composites are advantageous for N2 adsorption, as indicated by the nitrogen sorption isotherms discussed in the previous section. These large surface areas originate from the presence of zeolite with porous structure in nanocomposites. Therefore as a support for the cobalt ferrite nanoparticles, the ferrisilicate zeolite can provide more nitrogen molecules for the photocatalytic process. Moreover, as established by SEM and TEM images, the distribution of the cobalt ferrite particles on the surface of the zeolite effectively decreases the particle size of ferrite. This leads to the increment of photocatalytically active sites and boosts the surface area of CoFe2O4, which can promote the ammonia production. In addition, Co2+ and Fe3+ as transition metals with electrons in the d orbitals (ed) can activate the triple bond in N≡N through π backdonation (ed (Co2+, Fe3+)\(\to\) π* (N2))48 and thereby facilitate the photoreduction of N2. A comparative assessment of ammonia production rates for the optimal synthesized sample, photocatalysts with ferrite nanoparticles, and those containing Co and Fe sites can be found in Table 2. The data reveals that the CF50%/FS sample holds substantial potential in N2 photofixation among the literature.
Table 2 Comparison of photocatalytic ammonia production in the present study and previously reported photocatalysts.To investigate the recyclability of the catalysts, five runs were carried out under the identical reaction conditions. For this purpose, two of the best performed samples (CF25%/FS and CF50%/FS) were selected. After each cycle, the photocatalyst was separated from the solution, washed and dried to reuse. As can be seen in Fig. 13, the photocatalytic performances are decreased to 690.54 and 675.21 µmol L−1 after the fifth cycle for CF25%/FS and CF50%/FS, respectively.Fig. 13Recyclability of CF25%/FS and CF50%/FS photocatalysts.
Improvement of photocatalytic ammonia production of cobalt ferrite nanoparticles utilizing microporous ZSM-5 type ferrisilicate zeolite
