Abstract
Graphene quantum dots (GQDs) are emerging as environmentally friendly, low-cost, and highly tunable building blocks in solar energy conversion architectures, such as solar (fuel) cells. Specifically, GQDs constitute a promising alternative for organometallic dyes in sensitized oxide systems. Current sensitized solar cells employing atomically precise GQDs are based on physisorbed sensitizers, with typically limited efficiencies. Chemisorption has been pointed out as a solution to boost photoconversion efficiencies, by allowing improved control over sensitizer surface coverage and sensitizer-oxide coupling strength. Here, employing time-resolved THz spectroscopy, we demonstrate that chemisorption of atomically precise C42-GQDs (hexa-peri-hexabenzocoronene derivatives consisting of 42 sp2 carbon atoms) onto mesoporous metal oxides, enabled by their functionalization with a carboxylate group, enhances electron transfer (ET) rates by almost 2 orders of magnitude when compared with physisorbed sensitizers. Density functional theory (DFT) calculations, absorption spectroscopy and valence band X-ray photoelectron spectroscopy reveal that the enhanced ET rates can be traced to stronger donor–acceptor coupling strength enabled by chemisorption.
Highlights
Graphene quantum dots (GQDs) are nanosized graphene fragments, which have nonzero, size-dependent bandgaps due to quantum confinement effects.[1−3] graphene quantum dot (GQD) are metal-free and potentially low-cost and environmentally friendly
Employing time-resolved THz spectroscopy, we demonstrate that chemisorption of atomically precise C42-GQDs onto mesoporous metal oxides, enabled by their functionalization with a carboxylate group, enhances electron transfer (ET) rates by almost 2 orders of magnitude when compared with physisorbed sensitizers
GQDs are typically prepared by hydrothermal treatment of graphene or small molecules;[1−3] certain control of GQD size has been achieved following this synthesis protocol,[2] samples are generally defined by broad absorption features induced by inhomogeneous broadening, an aspect that is detrimental for optoelectronic applications
Summary
Onto metal oxides might improve photoconversion efficiencies in sensitized systems, there is at present no experimental evidence to support that claim. These results are in good agreement with previous values inferred for nanostructured SnO2 films[37−39] and, demonstrate that the monitored signal in Figure 2a refers uniquely to electrons populating the oxide conduction band These results support our conclusion that the changes in the monitored interfacial ET rates can be traced uniquely to the presence of the phenyl carboxylate group functionalizing the GQDC42-PhCOOH, allowing for chemisorption. When the interaction of the metal oxide with the molecules is not taken into account, upon sensitization, the slightly larger ΔG (the energy difference between the LUMO of the sensitizer and the CB of the electron-accepting oxide, see Figure 3) for the GQDC42 lacking a carboxylate group should lead to faster ET for the physisorbed sensitizers.[29] From the OPTP data shown, it is clear that this is not the case, which shows that the chemical interaction between chemisorbed GQDC42-PhCOOH and the oxide electrode affects donor−acceptor energetics (and coupling strength). Faster ET rates as a function of shorter donor−acceptor orbital distances between sensitizers and metal oxides have been demonstrated in quantum dot−oxide[41] and dye−oxide[40] systems
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