Conduction band photonic trapping via band gap reversal of brookite quantum dots using controlled graphitization for tuning a multi-exciton photoswitchable high-performance semiconductor

Abstract

Brookite exists as the metastable phase of titania and often mediates the transformation of anatase to rutile. The photocatalytic competence of brookite relative to polymorphs anatase and rutile has generally been considered structurally and energetically unfavourable for reasons that remain largely unknown and unchallenged. However, the process of phase transformation and performance related cooperativity among all three polymorphs has recently unlocked alternative directions for exploring brookite photovoltaics. Here, we demonstrate the programmable re-configuration of anatase to quantum confined reduced graphene (rGO)–brookite and show it is entirely modulated by surface-driven effects. Key components to this mechanism suggest that the self-assembly of rGO–brookite quantum dots is defect driven through pathways that favour a direct-to-indirect band gap reversal resulting from the graphitization of brookite. The accompaniment of new bandgap characteristics under quantum confinement introduce new hybridized energy states at the graphitic carbon–brookite juncture by modulation of the intrinsic sp² character to extrinsic sp³ clusters intermediate to graphene quantum dots (GQDs) and graphene oxide quantum dots (GOQDs). Evidenced by the intercalation of photochromic/fluorescent carbazole and anthracene moieties within the rGO framework by self-assembly, we show that the acquired fluorescence and luminescence properties of rGO–brookite are multi-emissive and reversibly quenchable under light excitation and from solvent polarity differences. Further, tuning the excitonic response of rGO–brookite by modulation of the photoluminescence (PL) signal intensity signifies coordinated interaction between localised carbazole and benz(a)anthracene moities which can undergo further structural refinement to adapt more optimally to both internal and external energy waves. Distinguishable by a large red-shift in the photoluminescent emission peak at λ_{479 nm} in the NIR region, we infer that a photoelectron sink driven by the quantum confinement of a narrow band gap of 0.78 eV formed from the orbital overlap of unoccupied interfacial sites promotes strong e⁻h⁺ coupling in the hybridized defect structure imposing a high charge separation by hindering e⁻h⁺ recombination. Modulation of interlayer spacing between rGO sheets and the synergy of complexation between intercalated carbazole/benz(a)nthracene can be adapted to achieve rapid photodegradation characteristics for DSSC applications.