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Inorganic Chemistry
pubs.acs.org/IC
Article
Quanta Master Fluorimeter with a Xe-arc lamp as the excitation
source. X-ray photoelectron spectroscopy (XPS) was performed on a
Kratos Axis Ultra using a monochromatic Al source. Collected spectra
were corrected for charging by referencing the C(1s) peak to 284.8
eV. All peaks were fitted in Casa XPS with the Shirley-type
background.
Photocatalytic Activity Tests. A 10 ppm solution of MB was
prepared in 18.2 MΩ water (Millipore). Reactions were run in 30 mL
quartz vials containing 20 mg of catalyst and 20 mL of MB solution.
All photocatalytic reactions were carried out in front of a Newport-
Oriel 150W Xe arc lamp solar simulator equipped with a 420 nm cut-
on filter and water filter. Vials placed in front of the lamp received a
front cell irradiance of 150 mW/cm2. Prior to irradiation, the
solutions were stirred for an hour in the dark to establish an
adsorption−desorption equilibrium. The reactions were then
illuminated. To monitor the reaction progression, 3 mL aliquots
were removed at 1 h intervals and centrifuged at 4000 rpm for 3 min.
The absorbance of the supernatant solution was monitored at 665 nm,
and the aliquot was then reintroduced to the reaction solution.
variability of this class of materials makes it even more ideal for
understanding the robust nature across multiple iterations.
In this work, we present the fast synthesis of crystalline
spinel MgFe2O4 through rapid microwave heating and
subsequent annealing. Nanocrystalline particles are achievable
through the commonly predicted organic reaction pathways
with the utilization of ethanol. Under visible light illumination,
the degradation of organic pollutants is possible on the surface
of the particles. Variable reactivity arises from changes to the
particles’ electronic and physical structures as the annealing
temperature increases.
EXPERIMENTAL SECTION
■
Materials and Methods. Magnesium acetylacetonate (TCI
America, >98%), iron(III) acetylacetonate (Strem Chemicals, 99%),
coumarin (Alfa Aesar, 98%), methylene blue trihydrate (MB), and
ethanol (200 proof) were purchased from Fisher. Tetramethylammo-
nium hydroxide pentahydrate (Acros Organics, 98%) was purchased
from Sigma-Aldrich. All other chemicals were used as received. All
procedures can be carried out safely with standard laboratory safety
and chemical hygiene training.
RESULTS AND DISCUSSION
■
Magnesium ferrite nanoparticles were obtained through rapid
microwave heating and subsequent annealing to impart
crystallinity. Prior literature examples showed that synthesis
attempted using precursor stoichiometry resulted in lingering
iron oxide impurities.19 As such, for this work, we employed an
excess of magnesium precursor to avoid the formation of these
impurities upon annealing. The powder X-ray diffraction
(PXRD) pattern for the resulting powders is shown in Figure
1. All of the diffraction peaks can be assigned to the spinel
Microwave Synthesis. In a typical synthesis, 0.9 mmol of
magnesium acetylacetonate and 1.0 mmol of iron(III) acetylacetonate
precursors were added to 5 mL of ethanol that was being stirred.
Separately, 2.0 mmol of tetramethylammonium hydroxide pentahy-
drate (Me4NOH) was dissolved in another 5 mL of ethanol. The two
solutions were mixed together in a glass microwave vial with an inner
volume of 30 mL, sealed with a PTFE-lined cap. The mixture was
sonicated for ∼15 min to fully dissolve the dispersed precursors upon
which a dark red solution was formed. During microwave heating,
stirring was maintained with a magnetic stir bar in the reaction
mixture at 800 rpm. The organic reaction was induced by automatic
adjustments to the microwave power to rapidly heat the solution to
180 °C (Figure S1). This temperature is held constant for 15 min
before being quenched with compressed air and cooled to room
temperature. The temperature and pressure are controlled and
maintained via an internal IR thermometer and pressure sensor,
respectively. After irradiation, the resulting precipitate is collected by
centrifugation and thoroughly washed with ethanol (3x). The
powders were dried in a vacuum oven (Fisher Scientific) at 60 °C
overnight. Samples were annealed at their specified temperatures for 1
h, achieved with a heating rate of 10 °C·min−1. All samples are
denoted by their annealing temperatures, T in °C.
Material Characterization. The microwave experiments were
conducted using an Anton Paar Monowave 400 synthesis reactor.
Powder X-ray diffraction data (XRD) were collected on a Panalytical
Empyrean diffractometer at a power of 1.8 kW (45 kV, 40 mA) with
Cu Kα (λ = 1.5418 nm) radiation. The detector was a X’Celerator
Scientific, a position sensitive 1D detector equipped with Bragg−
BrentanoHD X-ray optic delivering only Kα radiation. Patterns were
collected with a sampling step of 0.020, a scan rate of 0.080°·s−1, and
a rate of spinning at 0.25 Hz. Thermogravimetric analysis (TGA) was
performed on a TA Q50 TGA with a platinum pan, heating rate of 10
°C min−1, and a compressed air flow rate of 20 mL min−1.
Transmission electron microscopy (TEM) and energy dispersive X-
ray spectroscopy (EDX) was performed on a Thermo Fisher Talos
F200x G2 high-resolution electron microscope operated at 200 kV.
The samples were ground with a mortar and pestle and then
suspended in ethanol. One drop of this suspension was deposited on a
carbon grid, and the ethanol was allowed to evaporate before sample
imaging. Brunauer−Emmett−Teller (BET) surface area measure-
ments of the catalysts were obtained on a Quantachrome
NOVA4200e. A Varian Cary 5000 spectrophotometer equipped
with an external diffuse reflectance accessory was used for UV−vis
measurements. Spectra were collected in reflectance mode and
transformed into absorbance using the Kubelka-Monk function.
Barium sulfate was used as a 100% reflectance standard. Fluorescence
and photoluminescence (PL) spectra were collected using a Horiba
Figure 1. Powder XRD pattern of MgFe2O4 nanoparticles annealed at
600 °C (black), 700 °C (red), 850 °C (blue), and 1000 °C (green)
and reference pattern (vertical red lines) corresponding to spinel
MgFe2O4.
phase of MgFe2O4 (PDF #88-1942). Energy dispersive X-ray
spectroscopy confirms the ideal stoichiometry for the ternary
systems. The average domain size for each annealed sample
calculated using the Scherrer equation on the (311) reflection,
the inversion parameter from the Rietveld refinement, the
lattice constant, band gap, and surface area from N2 sorption
isotherms are presented in Table 1. Transmission electron
microscopy (TEM) of each sample agrees with the trend
observed of increased domain size as the annealing temper-
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Inorg. Chem. 2021, 60, 8704−8709