Nanospheres of Ag–coated Fe3O4 were successfully synthesized and characterized. Photocatalytic properties of Fe3O4@Ag composites have been investigated using steady state studies and laser pulse excitations. Accumulation of the electrons in the Ag shell was detected from the shift in the surface plasmon band from 430 to 405 nm, which was discharged when an electron acceptor such as O2, Thionine (TH), or C60 was introduced into the system. Charge equilibration with redox couple such as C60â-–/C60 indicated the ability of these core–shell structures to carry out photocatalytic reduction reactions. As well, outer Ag layer could boost charge separation in magnetic core through dual effects of Schottky junction and localized surface plasmonic resonance (LSPR)–powered band gap breaking effect under sunlight irradiation; resulted in higher photocatalytic degradation of diphenylamine (DPA). The maximum photocatalytic degradation rate was achieved at optimum amount of Ag–NP loading to products. Adsorption studies confirmed that degradation of DPA dominantly occurred in solution. Moderately renewability of the nanocatalysts under sunlight was due to oxidation and dissolution of the outer Ag layer.
KEYWORDS: Core–shell Fe3O4@Ag; Plasmonic photocatalysis; Laser pulse excitations; Charge equilibration; Schottky junction; Diphenyl amine
Core–shell nanocomposites combine the profitable properties of both the core and the shell materials (1). Various types of core–shell materials have been technically synthesized owing to their unique physicochemical properties and great potential applications (2,3). Among them, superparamagnetic core–shell nanocomposites do not retain any magnetization in the absence of a magnetic field (4). Hence, they have been broadly used in magnetic resonance imaging, hyperthermia, separation and purification of biomolecules, drug delivery, and catalysis (4,5).
The combination of nanocatalysts together with magnetic carriers has attracted increasing attention due to their recoverable nature from the mother solutions in presence of an appropriate magnetic field (6). Recently, to prevent the agglomeration and to further improve the durability of the nanocatalysts, various core–shell like magnetic chemcatalytic and photocatalytic nanomaterials have been developed (7–9).
Due to weighty role of Ag based magnetic nanocatalysts in fine and specialty chemistry, different kinds of this bi–functional nanostructures such as Fe3O4–Ag core–shell like NPs, heterodimers, and core–satellite particles have been prepared (11,12). The Ag component in most of the above products was located on the surface of the magnetic carrier whereas structures with an Ag core and Fe3O4 shell are rare.
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This article aims primarily to unravel the major mechanisms in magnetic core–shell plasmonic photocatalysis. It is important to elucidate the influence of the metal shell layer on the photoinduced charge separation in inner magnetic carrier and reveal the occurrence of charge equilibration between the metal and magnetic semiconductor. Therefore, we have prepared Fe3O4, Ag–coated Fe3O4 (Fe3O4@Ag) in ethanol medium and their behavior under UV–excitation were compared. The factors that control the charge separation and photocatalytic properties of coated nanostructures are also presented in this paper. Besides, we selected diphenylamine (DPA) as a model contamination (13–17) to present powerful and cost–effective photocatalysts. The European Union has listed DPA as a prior pollutant (14). According to the best of our knowledge, the photocatalytic degradation of DPA using Fe3O4@Ag nanospheres has not been reported, previously.
The operational conditions in photocatalytic removal of DPA were optimized. The effect of Ag–NPs loading on photocatalytic activity of core–shell nanoparticles was also investigated. Further studies were designed to answer the questions of whether DPA adsorbed on the Ag surface is an important step in its photocatalytic degradation rate or not? Eventually, tentatively reviews on the efficiency and durability of core–shell photocatalysts under sunlight irradiation were checked up.
Materials and Measurements
Powders of DPA, D(+)–glucose anhydrous, thionin acetate salt (C12H9N3S.C2H4O2), AgNO3 (99%), FeCl2.4H2O (>98%), FeCl3.6H2O (>99%), NH3.H2O (25–28%) and HPLC grade acetonitrile (purity 99%) were purchased from Sigma–Aldrich. The hexahydra salt CoCl2 was purchased from Riedel–de Haen Germany.
DPA was purified by simple preparative chromatography on a silica gel column (3:1 n–hexane/acetonitrile as a mobile phase) and followed by thin layer chromatography (TLC) monitoring. All other materials were of highest purity commercially available and were applied without further purification.
The Britton–Robinson buffer solutions were prepared in 0.04 M concentration. The DPA stock solution was set up by dissolving 10.0 mg of the powders in 100 mL of 60/40 v/v buffer solution/acetonitrile and then stored in a refrigerator. High purity water purified with the Milli–Q system was used in all experiments.
The transmission electron microscopy (TEM) study was carried out using a Hitachi S–4300 (Japan) instrument. The crystalline structure of the powders was studied by X–ray diffraction (XRD) with a PHILIPS PW–1840 diffractometer. The UV–vis spectra were recorded on a Biotech Diode–Array spectrophotometer. The IR spectra of the synthesized magnetic NPs were obtained using a Shimadzu FT–IR 8300 spectrophotometer. Magnetic measurements were made with a Quantum Design PPMS Model 6000 magnetometer at 25 °C. The pH values of all solutions were assessed by a model 744 Metrohm pH meter (Switzerland). An external magnet bar of 5 cm×5 cm×3 cm and power of 1.46 T was used for the accumulation of magnetic NPs. The photodegradation of DPA has been monitored using UV–vis spectrophotometer (Biotech) and a HPLC (KNAUER).
The HPLC system used throughout this study consisted of a HPLC pump (KNAUER, K–1001, USA), a sample injector with a 100 ïL loop and a UV detector (KNAUER, K–2600). The column used was a reversed–phase Spherisorb C18 column (250 mm × 4.6 mm i.d., 5 ïm). The mobile phase was acetonitrile–water (65:35 v/v) with a flow–rate of 1.0 mL/min. The column temperature was 25 °C. The effluent was monitored at 254 nm.
Preparation of Fe3O4@Ag nanoparticles
Fe3O4–NPs were prepared using the most conventional reported co–precipitation method first (18), followed by the slow reducing of the Ag+ ions to form a metal shell around the core. Calculated amount of freeze dried magnetic NPs were well–dispersed in 10 mL deionized water. A 10.0 mL portion of 1.0 mM AgNO3 solution was then added into suspension. Glucose was used as a mild reducing agent for the reduction of Ag+ ions (19). Increasing the amount of glucose increases the reduction rate of Ag+ ions. We have found that the experimental conditions that employ molar ratio of metal ions to glucose of 2:1 yields stable suspension of core–shell particles. The condensation deposition of metal particles slowly progresses to yield ~2–3 nm metal shell. With continued stirring of the solution at room temperature, the color slowly changed from black to brownish. Optimized reaction time of ~25 min was achieved based on maximum photocatalytic activity of core/shell clusters. Ag–NPs were also produced in a separate batch using the same experimental conditions.
Laser Flash Photolysis
Experiment of nano–second laser flash photolysis was performed with 337 nm laser pulses from N2 laser system (Laser pulse width 800 ps, intensity 5 mJ/pulse). Unless otherwise specified, all the experiments were performed under N2 purging condition. Steady–state photolysis experiments were conducted by photolyzing N2–purged solution with UV light (two high–pressure 15 W mercury lamps).
The adsorption and photocatalytic degradation of DPA was carried out in a home–made cylindrical Pyrex reactor (50 mL) with a double–walled cooling–water jacket. UV illumination was conducted utilizing two UV lamps housed over the photocatalytic reactor. In all the experiments, the reactor was fixed 15 cm distant from the light sources. Prior to illumination, equal volumes of DPA and photocatalyst suspension (50 mL volumes) were stirred in the dark for 15 min to achieve the adsorption–desorption equilibrium. Then, UV–irradiated samples (3 mL) were obtained at fixed time intervals and exposed to an external magnetic field for separation of photocatalysts from the reaction mixture. Sample analysis was done by recording the UV–vis absorbance spectra and, simultaneously, injecting of 10 ïL of solution into the HPLC column. The kinetic data are presented as means of triplicate experiments.
Results and discussion
Characterization of the prepared nanoparticles
The studies of size, morphology and composition of the NPs were performed by means of TEM images, FTIR spectra, XRD patterns, UV–vis absorption spectra and magnetization tests. The TEM images of the core–shell clusters demonstrate that these particles have spherical shape with average size of 9.0±2.0 and 12.0±2.0 nm, respectively (Figure 1A and 1B). Figure 1B shows that a pale shell was coated on the surface of the black core and the interface between the core and shell is sharp and clear. The surface of the core–shell particle is rather rough. The particle size analysis illustrates that the Fe3O4 particles are coated with silver (Figure 1C and 1D).
The change of absorption peaks in the FTIR spectra indicate that the Ag–NPs are coated on the surface of Fe3O4–NPs (Figure S-1A) (20). The absence of characteristic diffraction peaks of Fe3O4 reflection in the XRD pattern manifests complete coating of the Fe3O4 seeds by Ag metal (Figure S-1B) (21). After reduction of Ag ions, a new strong absorption band in the UV–vis absorption spectra is observed at 420 nm, which is assigned to the surface plasmon resonance peak of Ag–NPs (Figure S-1C) (22). The large decrease in the magnetic moment of the Fe3O4–NPs after coating with Ag–NPs is attributed to the presence of nonmagnetic Ag metal in the prepared composites (Figure S-1D) (19).
Figure 2A shows the changes in the absorption spectrum following the UV–irradiation of Fe3O4@Ag colloids suspended in de–aerated ethanol as a steady–state photolysis. Before subjecting to UV–irradiation, the plasmon absorption peak of suspension is seen at 430 nm. It should be noted that the small Ag particles prepared using glucose reduction represent absorbance peak at around 420 nm (19,22). The red shift in the plasmon absorption of the core–shell particles is dependent on the type of the oxide contact layer, refractive index of the surrounding medium, the volume fraction of shell layer (23), scattering effects and adsorbed chemical species (24).
For 15 min UV–irradiated sample, the absorption shift attains a plateau with a surface plasmon absorption peak at 405 nm (25). For comparison, no spectral shift was observed during the UV–irradiation of bare Ag–NPs suspension in ethanol (Figure 2B).
Transient absorption studies were probed using nanosecond laser flash photolysis (Figure S-2A). Notably, the spectral feature of the transient spectrum (Figure S-2A) closely matches with the difference spectrum recorded in steady–state photolysis as shown in the inset of Figure 2A. We can also repeat the photoinduced charging and dark discharge cycles repeatedly and reproduce the plasmon absorption response to separated electrons (Figure S-3) (24).
Estimation of the amount of Electrons accelerated into Ag shell layer
Known amounts of concentrated thionine solution (degassed) as a redox couple was injected in small increments into the UV–irradiated Fe3O4@Ag suspension (24). The absorption spectrum was recorded after each addition of thionine (Figure 3A). The presence of any unreduced thionine as the endpoint of titration is marked by the appearance of 600 nm absorption band. The plasmon shift can thus be related to the concentration of thionine added (inset of Figure 3A). From the slope of this linear plot until endpoint and the net shift observed in the plasmon band, we expect a maximum access of about ~35 electrons per Fe3O4@Ag core–shell particle (24). The dependence of the plasmon shift and the number of electrons versus the UV–irradiation time is also shown in Figure 3B.
We also selected C60 as an excellent probe to investigate interfacial electron transfer in colloidal core–shell magnetic systems (24). The absorption maximum at 1075 nm manifests formation of C60 anion (C60â-–) (Figure 4) (24). The electron transfer yield increased initially with increasing concentration of C60 (inset of Figure 4).
Photocatalytic activity of Fe3O4@Ag particles
The UV–vis absorption spectroscopy and HPLC experiments were performed to follow the photodegradation reaction progress. Figure 5A exhibits the changes in the absorbance spectra of DPA after black–light irradiation in the absence and presence of the nanocatalysts. Photographs from the solution of DPA before and after its photocatalytic degradation are shown in the inset of this Figure.
Figure 5B displays the photodegradation monitoring of DPA by HPLC. The separation method of DPA, intermediates, and products was very similar to those reported in literature (26). By irradiation of DPA with UV light for 40 min, a reduction in the chromatogram at 10.5 min in accompanying with the appearance of a new peak at a retention time of 9.3 min is observed. The obtained chromatograms suggest higher photodegradation rate of DPA in the presence of the Fe3O4@Ag clusters (Figure 5B).
The photocatalytic degradation kinetic results of DPA are shown in Figure 5C which can be well described by Langmuir–Hinshelwood (L–H) model (27). The rate constant, the linear plots of −ln(C/C0) vs. time was calculated as 0.041 min−1 for the coated particles (Figure 5D).
After maintaining DPA–NPs suspension in dark no new peak was appeared in the chromatogram (plots (a) and (b) in Figure 5C). Using surface enhanced Raman scattering (SERS) sensing, Du and Jing showed that oxidation of the aromatic compounds containing a free electron pair on the nitrogen atom is increased using a modified Fe3O4@Ag magnetic NPs probe (28). Figure S-4A exhibits a Langmuir type adsorption isotherm of DPA (29).
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The effect of initial concentration of pollutant, pH, catalyst concentration, and shell coating time on the photodegradation rate of DPA were also investigated (30,31). Photocatalytic degradation rate constant of DPA is inversely proportional to its initial concentration which implies that the reaction dominantly occurred in solution rather than in the catalyst surface (inset of Figure S-4A) (30). The L–H equation also was successfully used to describe that DPA adsorbed on the Ag surface is not an important step (32).
Capping of Ag shell on the Fe3O4 core was confirmed by checking the stability in an acidic solution (HNO3). At pH <3, core–shell nanospheres were unstable. The minimal sedimentation of magnetic NPs in coated samples could be explained by Ag coating of the Fe3O4–NPs surface (33).
Significant shifting (~2nm) in spectra for DPA was detected at different pH values. Figure S-4B shows that the adsorption of DPA on Ag surface decreases, but the removal of DPA increases with the increasing pH. At sufficiently higher pH values, the formation of oxidizing species such as the oxide radical anion (â-O–) could also be responsible for the enhancement (34). The observed results are consistent with the proposed mechanism for the photolysis of DPA in literature (35).
Figure S-4C shows the time–dependent degradation of DPA at different concentrations of nanocatalysts (36). At excess concentrations of nanocatalysts, considerable decreasing in the photocatalytic activity can be attributed to the low probability of provoking all photocatalysts in solution together with their self–absorption effects.
The photocatalytic activity of Fe3O4@Ag clusters initially increases to a peak and then decreases with increasing coating thickness (Figure S-4D), most possibly due to shading (37–39), strong scattering and light filtering effect (40) of denser coating. Varying the Ag shell thickness and the refractive index of the solvent allows control over the optical properties of the dispersions (inset of Figure S-4D) (41).
After 40 min photocatalytic reaction, core–shell nanocatalysts were collected by using a small magnet followed by twice washing with deionized water for reusing (Figure 6). In the first cycle of sunlight irradiation, ~95% degradation of DPA was achieved. However, after 3 recycling reactions, photocatalytic activity of the coated particles greatly reduced to the activity level of bare Fe3O4–NPs. Corrosion (38,42,43), oxidation (42,44) or dissolution of the noble metal coating are likely to limit the use of noble metals (Figure S-5A and S-5B).
Moreover, the absence of holes in the outer layer of the core–shell particles was investigated. After each addition of known amounts of concentrated Co2+ solution into the UV–irradiated Fe3O4@Ag suspension no color change was observed (Figure S-5C and S-5D).
A series of ROSs, such as â-OH, â-O2−, â-HO2 and H2O2, are subsequently produced from primary active photogenerated holes and electrons (30). 0.1 M isopropanol or sodium azide (NaN3) was added in the reaction solution as scavengers of â-OH radicals (45). I− ions was selected to scavenge the photoholes and resulted â-OH radicals by forming relatively inert iodine radicals (30,46). The obtained pseudo–first–order rate constants with or without the addition of various scavengers are all presented in Table 1.
In the presence of isopropanol and NaN3, the pseudo–first–order rate constants decreased from 0.041 min−1 to 0.014 and 0.017 min−1, respectively. The degradation rate of DPA with ~65.0% yield is contributed by the â-OH radicals. Comparatively, the rate constants also decreased very closely to 0.018 min−1 after addition of KI scavengers in the reaction solution. Thus, the contribution percentage of photoholes in the degradation rate was deduced as ~0%. Photocatalytic degradation rate constant of deaerated DPA solution with N2 was roughly stopped, since moved electrons toward the outer layer don’t receive oxygen. Therefore, only 35.0%, of the degradation rates were from other ROSs or direct photolysis of DPA.
We have scrutinized the photoinduced charging and dark discharging of electrons in a magnetic core–silver shell structure. The shift in surface plasmon band serves as a measure to determine the number of electrons accelerated into the metal shell. The charge equilibration between the metal and magnetic semiconductor plays a significant role in dictating the overall energetic of the composite. These magnetic core–metal shell composites are photocatalytically active and are practical to promote light induced electron–transfer reactions. The enhanced sunlight photocatalytic activity of nanocomposite could be attributed to a synergistic effect between LSPR–powered bandgap breaking effect and bandgap–excitation effect modes (38,47–52). In this photocatalytic system, presence of oxygen for starting the degradation of pollutants is imperative. Exploring the catalytic activity of such composite structures could pave the way for designing novel light harvesting systems.
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