Figure S-1. (A) FTIR spectra of (a) Fe3O4 and (b) Fe3O4@Ag–NPs. (B) X–ray diffraction
pattern of (a) Fe3O4 and (b) Fe3O4@Ag–NPs prepared with reaction time of 25 min. (C) UV–
vis absorption spectra of (a) Fe3O4, (b1→b8) Fe3O4@Ag–NPs at different coating times with
5–min intervals and (c) Ag. (D) Hysteresis loops recorded at room temperature of (a) Fe3O4
and (b) Fe3O4@Ag–NPs. Inset shows photograph of the Fe3O4@Ag–NPs suspension after
magnetic separation by an external magnet.
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Figure S-1A reveals the FTIR spectra of Fe3O4–NPs and core–shell Fe3O4/Ag nanospheres.
As seen, the stretching frequency at 619 cm–1 is assigned to the Fe–O bond of Fe3O4. For the
silver–coated sample, the absorption peak assigned to Fe–O bond is blue–shifted from 619 to
578 cm–1. This change of absorption peaks indicate that the Ag–NPs are coated on the surface
of Fe3O4–NPs. These results further confirm that the Fe3O4–NPs are not coated with glucose.
The XRD pattern of the as–prepared Fe3O4 seeds is shown in Figure S-1B. These
peaks matched well with standard Fe3O4 reflections, which confirm the formation of Fe3O4
seeds. The resulting XRD pattern for reduction of Ag ions for 25 min as an optimum time in
the presence of Fe3O4 seeds is also included in Figure S-1B. The XRD pattern clearly
discloses four diffraction peaks at 38.1°, 44.3°, 64.4°, and 77.4°, corresponding to (111),
(200), (220), and (311) planes of silver, respectively. This result further suggests the
formation of Fe3O4@Ag nanospheres. At the same time, the absence of characteristic
diffraction peaks of Fe3O4 reflection manifests complete coating of the Fe3O4 seeds by Ag
metal, providing a convincing evidence for thorough coverage of the Fe3O4–NPs by Ag and
indicating that Fe3O4@Ag nanospheres can be prepared at high yield by the present simple
method. As the Ag content increased, the intensity of the diffraction peaks decreased and a
broadening of the full width at half the maximum (FWHM) of the peaks was also noticed
(inset of Figure S-1B). This can be attributed to an increase in amorphous content and
diminution of grain size with the addition of Ag. Based on the Debye–Scherrer equation, for
the (111) peak with 2θ at 38.1°, the average crystalline sizes of the core–shell samples were
obtained approximately 12±2 nm. Although the presence of nanocrystal defects and
polycrystallinity may cause inaccurate estimations by the Debye–Scherrer equation, the
repeated XRD analysis showed similar results with different batches of nanospheres
produced and the XRD particle size estimation is in good agreement with the TEM result.
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Formation of Fe3O4@Ag nanospheres was further supported by the UV–vis
absorption spectra. Figure S-1C presents the absorption spectra of Fe3O4, Fe3O4@Ag and
pure Ag–NPs. There were no significant absorption peaks attributable to Fe in the visible
region of the pure Fe3O4–NPs solution spectra. However, in the spectra for Fe3O4@Ag–NPs
solutions, surface plasmon peaks appeared in the 400–450 nm range, indicating the formation
of an Ag shell. As the coating time of Fe3O4–NPs solution increased, the surface plasmon
peak observed for Fe3O4@Ag–NPs was blue–shifted from 450 nm to 400 nm. Further, the
sharpness of the peak increased and the breadth of the band decreased as coating time of
Fe3O4–NPs solution increased. The observed peak broadness may be caused by non–uniform
Ag shells around the Fe3O4 cores. Higher volumes of Fe3O4–NPs solution are likely to cause
formation of smaller Fe3O4@Ag–NPs.
Figure S-1D exhibits the magnetization plotted as a function of applied field (M
versus H curves). It can be seen that the M–H curves exhibit nonlinear, reversible behavior
with observed coercivity and remanence. It was found that the magnetization and coercivity
of Fe3O4–NPs are 29.0 emu/g. For the silver–coated sample prepared with a reaction time of
25 min, the magnetization further decreased to 24.0 emu/g. The large decease 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.
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Figure S-2. (A) Difference absorption spectra of deaerated colloidal Fe3O4@Ag (○) recorded
following 337 nm laser pulse excitation. Representative absorption–time profiles at (B) 405
nm and (C) 460 nm (experimental conditions are the same as in Figure S-2A).
Transient absorption studies and the photoinduced charge–transfer events were also probed
using nanosecond laser flash photolysis. The transient absorption spectra recorded following
337 nm laser pulse excitation of deaerated colloidal Fe3O4@Ag suspension is shown in
Figure S-2A. The Fe3O4@Ag nanospheres exhibit strong bleaching in the plasmon absorption
region in addition to the absorption in the red. The absorption band at 405 nm and bleaching
at 460 nm is the result of shift in the plasmon band following the electron movement to outer
Ag layer. It is interesting to note that the spectral feature of the transient spectrum (Figure S4
2A closely match the difference spectrum recorded in steady–state photolysis as shown in the
inset of Figure 2A). This observation indicates that the absorption change observed in steady–
state photolysis is a spontaneous process that follows the excitation of the magnetic core. The
transient absorption–time profile (Figure S-2B and S-2C) shows that most of the charge
transfer from excited Fe3O4 to Ag is completed within the laser pulse duration of few
nanoseconds.
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Figure S-3. Response of plasmon absorbance peak to electron storage following the UV–
irradiation of deaerated Fe3O4@Ag colloidal suspension in ethanol and dark discharge in air.
We also checked the reproducibility of charging and discharging of Fe3O4@Ag system by
repeated cycles of UV–irradiation of de–aerated suspension followed by exposure to air.
Figure S-3 shows the reproducibility of plasmon absorption peak response to the UV–
irradiation and air exposure in dark. The plasmon absorption band shifts from 430 to 405 nm
during 1min UV–irradiation of de–aerated Fe3O4@Ag suspension. The plasmon absorption
regains the original spectral features when the accelerated electrons are discharged in dark by
exposing to air. We can repeat the photoinduced charging and dark discharge cycles
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repeatedly and reproduce the plasmon absorption response to separated electrons. The
suspension was de–aerated for 15 min before UV–irradiation of each charging cycle.
Figure S-4. (A) Langmuir isotherm for DPA, 1 g/L Fe3O4@Ag catalyst, pH=4. Inset: Effect
of initial concentration of DPA on its photocatalytic degradation rate constant with 50 mg/L
Fe3O4@Ag concentration and pH value 7.0 (A0 and A denote maximum UV–vis absorbance
spectra of DPA before and after 15 min exposing by Fe3O4@Ag–NPs in dark). (B) Effect of
pH values on the photocatalytic degradation rate of 100 mg/L DPA with 150 mg/L
Fe3O4@Ag photocatalyst. Inset: Effect of pH on the adsorption of DPA on catalyst surface
(A0 and A are the same with Figure S-4A). (C) Effect of Fe3O4@Ag dosage on the
photocatalytic degradation rate of 100 mg/L DPA and pH value 7.0. (D) Photocatalytic
degradation rate of 100 mg/L DPA with 50 mg/L Fe3O4@Ag photocatalyst formed at
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different coating times. Inset: Variation of the measured light scattering to light absorption
ratio (CSca/CAbs) as a function of coating times or nanosphere photocatalyst size.
Figure S-5. Photographs represent Fe3O4@Ag suspension (A) before UV irradiation, (B)
after adding NaCl to the 30 min pre–UV–irradiated sample under O2 saturated conditions,
injecting concentrated Co2+ solution (degassed) into the 30 min pre–UV–irradiated (C)
Fe3O4@Ag suspension under deaerated conditions, (D) Fe3O4 suspension under deaerated
conditions.
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