Photocatalyzed electron exchange between organic chromophores and hematite nanoparticles and the role of solid-state charge transport

Abstract

An increase in the fluorescence lifetime of rhodamine B dye, when in contact with hematite nanoparticles in methanol from mildly acidic to alkaline pH, is consistent with the transfer of electrons from excited dye to the hematite conduction band.

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“…Methanol was used as the solvent in this system, as opposed to water, because it promotes better sorption of RhB onto hematite by interacting less strongly with the carboxyl groups of RhB. ,,, We tested for any XAS-detectable solvent effects on Fe valence for the HNPs, comparing suspensions in both methanol and water. To do this, we used a laboratory-based benchtop XAS system stationed at the Pacific Northwest National Laboratory.…”

“…After evaluating the TRXAS of HNPs in methanol alone, we examined cases with RhB added to the suspension at various pH values. Building on our previous work, using UV–vis measurements, we tested the pH-dependent RhB uptake on hematite in methanol (see SI, Figures S9–10, Text S4). Various forms of hematite have different point of zero charge (PZC) values ranging from pH 5.5 to pH 9. ,, For near-perfect single crystals, the point of zero net charge of low index facets of hematite is between pH 8 and 9 in water.…”

“…58 By using RhB as a probe, at pH below the point of zero charge of hematite surfaces, we observed an increase in its fluorescence lifetime consistent with transient electron transfer into the hematite. 46 For the acidic to mildly alkaline pH values that were examined, after normalizing the observed fluorescence decay kinetics to the measured surface loading of the RhB dye, our results indicated a surprising insensitivity of this process to environmental conditions. Thus, we concluded that the backreaction step was controlled instead by properties of the particles that control the mobilities of electron charge carriers, i.e., small polaron hopping along the Fe sublattice.…”

“…Although in our previous work using TAS, we examined suspensions in both water and methanol, here we focused exclusively on suspensions in methanol due to weaker solvent-RhB interactions and readily identifiable photocatalyzed interfacial electron transfer conditions. 46 HNP suspensions with a total Fe 2 O 3 concentration of 8 mM were prepared by sonicating a measured (128−153 mg) amount of hematite nanoparticles in ∼100−120 mL of methanol for at least 1 h. The sonicated suspension was allowed to settle for ∼20 min to allow bigger particles/aggregates to segregate at the bottom of the container. After the suspension had settled, roughly 80% of the suspension was extracted from the top for experiments.…”

“…Thus, we concluded that the backreaction step was controlled instead by properties of the particles that control the mobilities of electron charge carriers, i.e., small polaron hopping along the Fe sublattice. 46 However, this work focused on electron transfer and recombination kinetics from the perspective of the dye and could not detect transient electrons injected into hematite.…”