Evaluating carbon dots as electron mediators in photochemical and photocatalytic processes of NiFe2O4

Abstract

Spinel ferrites such as nickel ferrite are promising energy conversion photocatalysts as they are visible-light absorbers, chemically stable, earth abundant, and inexpensive. Nickel ferrite shows poor photocatalytic activity due to fast electron–hole recombination upon illumination. This study evaluates the capability of carbon dots (CDs) to improve charge-carrier separation in NiFe2O4. We report a facile solvothermal approach for synthesizing NiFe2O4 and CDs/NiFe2O4 nanoparticles at 200–215 °C. The photocatalysts were characterized using transmission and scanning electron microscopy, x-ray diffraction, Raman spectroscopy, Fourier transform infrared spectroscopy, UV-VIS-NIR spectroscopy, photoelectrochemical analysis, and laser flash photolysis. Photocatalytic oxidation of methanol to formaldehyde under visible light was employed to test the effect of CDs on the photocatalytic efficacy of NiFe2O4. UV-VIS-NIR spectroscopy depicted a total quenching of NIR absorption and a diminished absorption of a peak at ∼745 nm in CDs/NiFe2O4 compared with NiFe2O4, indicating a transfer of electrons from NiFe2O4 to CDs. A 12-fold increment in the incident-photon-to-charge-efficiency was achievable with CDs/NiFe2O4 (0.36%) compared with NiFe2O4 (0.03%). Impedance spectroscopy exhibited a more efficient charge separation and faster interfacial charge transfer in CDs/NiFe2O4 compared with pure NiFe2O4. This was accounted for by the lower initial quantity of charge carrier upon irradiation in CDs/NiFe2O4 compared with NiFe2O4 as detected from laser flash photolysis, indicating that CDs acted as electron acceptors and reservoirs in CDs/NiFe2O4. Compared with NiFe2O4, CDs/NiFe2O4 showed an enhanced photocatalytic activity toward formaldehyde formation. Consequently, CDs are good electron mediators for NiFe2O4, capable of improving charge-carrier separation and the photocatalytic activity of NiFe2O4.

Magnetic NiFe2O4 Nanoparticles Prepared via Non‐Aqueous Microwave‐Assisted Synthesis for Application in Electrocatalytic Water Oxidation

“…Peaks appearing at 2980-2930 cm À 1 are due to CÀ H vibrations. [36,41,45] The presence of surface hydrocarbons is additionally indicated by TGA-MS of as-synthesized samples (Figure 2e and f), revealing a significant mass loss of nearly 15 % in the heating range of 250-300 °C. Still, in the range of 300-500 °C samples lose another 2 % of their masses.…”

“…Notably, the cationic distributions in tetrahedral and octahedral sites can also be estimated using this technique if high quality data can be obtained. [41] Figure 2b shows the expected five active Raman modes of spinel-type NiFe 2 O 4 , even before calcination: T 2g (200 cm À 1 , 475 cm À 1 , 565 cm À 1 ), E g (320 cm À 1 ), and A 1g (685 cm À 1 ). [42] The most intense A 1g signal can be assigned to metal-oxygen symmetric stretching modes located in tetrahedral sites.…”

“…The phase‐purity of the prepared nanocrystals was additionally investigated by Raman spectroscopy, since small amounts of α ‐Fe 2 O 3 might not be detected via PXRD due to the broad reflections caused by the nanosized crystals or the potentially amorphous nature of this oxide. Notably, the cationic distributions in tetrahedral and octahedral sites can also be estimated using this technique if high quality data can be obtained [41] . Figure 2b shows the expected five active Raman modes of spinel‐type NiFe 2 O 4 , even before calcination: T 2g (200 cm −1 , 475 cm −1 , 565 cm −1 ), E g (320 cm −1 ), and A 1g (685 cm −1 ) [42] .…”

“…Signals at 2350–2340 cm −1 , 3500–3000 cm −1 , and 3700–3690 cm −1 can be attributed to carbon dioxide, water, and O−H surface vibrations, respectively. Peaks appearing at 2980–2930 cm −1 are due to C−H vibrations [36,41,45] …”

Nitrogen/Carbon-Coated Zero-Valent Copper as Highly Efficient Co-catalysts for TiO2 Applied in Photocatalytic and Photoelectrocatalytic Hydrogen Production

“…Copper nanoparticles (Cu1, Cu2, and Cu3) were synthesized by the solvothermal decomposition of Cu­(acac) 2 in either an oxygen- or nitrogen-containing surfactant. The utilized protocol was a modification of our previously reported solvothermal procedure, developed for the synthesis of nickel ferrite nanoparticles . In brief, 500 mg of Cu­(acac) 2 was suspended in 14 mL of triethylene glycol in a sealable pressure tube.…”