Surface-state-mediated interfacial charge dynamics between carbon dots and ZnO toward highly promoting photocatalytic activity

Substitutional Cd Dopant as Photohole Transfer Mediator Boosting Photoelectrochemical Solar Energy Conversion of 2D Cd-ZnIn2 S4 Photoanode

Fast recombination dynamics of photocarriers competing with sluggish surface photohole oxidation kinetics severely restricts the photoelectrochemical (PEC) conversion efficiency of photoanode. Here, a defect engineering strategy is developed to regulate photohole transfer and interfacial injection dynamics of 2D ZnIn2 S4 (ZIS). Via selectively introducing substitutional Cd dopant at Zn sites of the ZIS basal plane, energy band structure and surface electrochemical activity are successfully modulated in the Cd-doped ZIS (Cd-ZIS) nanosheet array photoanode. Comprehensive characterizations manifest that a shallow acceptor level induced by Cd doping and superior electrochemical activity make surface Cd dopants simultaneously act as capture centers and active sites to mediate photohole dynamics at the reaction interface. In depth photocarrier dynamics analysis demonstrates that highly efficient photohole capture of Cd dopants brings about effective space separation of photocarriers and acceleration of surface reaction kinetics. Therefore, the optimum 2D Cd-ZIS achieves excellent PEC solar energy conversion efficiency with a photocurrent density of 5.1 mA cm-2 at 1.23 VRHE and a record of applied bias photon-to-current efficiency (ABPE) of 3.0%. This work sheds light on a microstructure design strategy to effectively regulate photohole dynamics for the next-generation semiconducting PEC photoanodes.

Photocatalytic Degradation of Methylene Blue Using N-Doped ZnO/Carbon Dot (N-ZnO/CD) Nanocomposites Derived from Organic Soybean

This study reports on successful synthesis of carbon dots (CDs), nitrogen-doped zinc oxide (N-ZnO), and N-ZnO/CD nanocomposites as photocatalysts for degradation of methylene blue. The first part was the synthesis of CDs utilizing a precursor from soybean and ethylenediamine as a dopant by a hydrothermal method. The second part was the synthesis of N-ZnO with urea as the nitrogen dopant carried out by a calcination method in a furnace at 500 °C for 2 h in an N2 atmosphere (5 °C min-1). The third part was the synthesis of N-ZnO/CD nanocomposites. The characteristics of CDs, N-ZnO, and N-ZnO/CD nanocomposites were analyzed through Fourier transform infrared (FTIR), UV-vis absorbance, photoluminescence (PL), high-resolution transmission electron microscopy (HR-TEM), X-ray diffraction (XRD), thermal gravimetry analysis (TGA), field-emission scanning electron microscopy energy-dispersive spectroscopy (FESEM EDS), X-ray photoelectron spectroscopy (XPS), and Brunauer-Emmett-Teller (BET) analysis. Based on the HR-TEM analysis, the CDs had a spherical shape with an average particle size of 4.249 nm. Meanwhile, based on the XRD and HR-TEM characterization, the N-ZnO and N-ZnO/CD nanocomposites have wurtzite hexagonal structures. The materials of N-ZnO and N-ZnO/CD show increased adsorption in the visible light region and low energy gap E g. The E g values of N-ZnO and N-ZnO/CDs were found to be 2.95 and 2.81 eV, respectively, whereas the surface area (S BET) values 3.827 m2 g-1 (N-ZnO) and 3.757 m2 g-1(N-ZnO/CDs) belonged to the microporous structure. In the last part, the photocatalysts of CDs, N-ZnO, and N-ZnO/CD nanocomposites were used for degradation of MB (10 ppm) under UV-B light irradiation pH = 7.04 (neutral) for 60 min at room temperature. The N-ZnO/CD nanocomposites showed a photodegradation efficiency of 83.4% with a kinetic rate of 0.0299 min-1 higher than N-ZnO and CDs. The XRD analysis and FESEM EDS of the N-ZnO/CDs before and after three cycles confirm the stability of the photocatalyst with an MB degradation of 58.2%. These results have clearly shown that the N-ZnO/CD nanocomposites could be used as an ideal photocatalytic material for the decolorization of organic compounds in wastewater.

Integrating Photoluminescence and Ferromagnetism in Carbon Quantum Dot/ZnO by Interfacial Orbital Hybridization for Multifunctional Bioprobes

Integrating ferromagnetism (FM) and photoluminescence (PL) into one particular nanostructure as biological probe plays an irreplaceable role in accurate clinical diagnosis combining magnetic resonance and photoluminescence imaging technology. However, magnetic emergence generally needs a spin polarization at Fermi level to display a half-metallic electronic feature, which is not beneficial for preserving radiation recombination ability of photo-excited electron-hole carriers. To overcome this intrinsic difficulty, we propose a feasible atomic-hybridization strategy to anchor carbon quantum dots (CQDs) onto ZnO microsphere surface via breakage of C=O bonds at CQDs and subsequent Zn-3d and C-2p orbital hybridization, which not only ensures the carrier recombination but also leads to a room-temperature magnetism. Herein, the photoluminescence and magnetism coexist in this multifunctional heterojunction with outstanding biocompatibility. This work suggests that integration of magnetism and photoluminescence could be accomplished by particular interfacial orbital hybridization.

Insight into the Modulation of Carbon-Dot Optical Sensing Attributes through a Reduction Pathway

Oxidized/reduced carbon dots (CDs) with tunable optical features have emerged as a new class of CDs having a common "molecular origin" but different fluorescence (FL) behaviors. In the present work, using "banana peel" as a sole carbon source followed by doping with fluorine (F), boron (B), and nitrogen (N) over CDs, banana peel-derived carbon dots (BP-CDs) were synthesized using a well-known hydrothermal synthesis method. Moreover, as-synthesized BP-CDs were further reduced to "rBP-CDs" by NaBH4. At post reduction, the FL performance (i.e., quantum yield) of rBP-CDs were found to be enhanced compared with the BP-CDs, along with variations in excitation and emission wavelengths. Interestingly, the optical sensing attributes of BP-CDs and rBP-CDs were varied, that is, BP-CDs selectively sense "Co2+ with a limit of detection (LOD) value of 180 nM", whereas rBP-CDs detected Co2+ (with an LOD value of 242 nM) as well as Hg2+ (with an LOD value of 190 nM). To the best of our knowledge, this work presents the very first report on the modulation of CDs' sensing behavior after reduction. The modulation in the sensing behavior with the common carbon precursor and reduction paves a new possibility for exploring CDs for different commercial applications.

Nanocatalysis under Nanoconfinement: A Metal-Free Hybrid Coacervate Nanodroplet as a Catalytic Nanoreactor for Efficient Redox and Photocatalytic Reactions

Nature utilizes cellular and subcellular compartmentalization to efficiently drive various complex enzymatic transformations via spatiotemporal control. In this context, designing of artificial nanoreactors for efficient catalytic transformations finds tremendous importance in recent times. One key challenge remains the design of multiple catalytic centers within the confined space of a nanoreactor without unwanted agglomeration and accessibility barrier for reactants. Herein, we report a unique blend of nanoscience and chemical catalysis using a metal-free hybrid synthetic protocell as a catalytic nanoreactor for redox and photocatalytic transformations, which are otherwise incompatible in bulk aqueous medium. Hybrid coacervate nanodroplets (NDs) fabricated from 2.5 nm-sized carbon dots (CDs) and poly(diallyldimethyl)ammonium chloride have been utilized toward reductive hydrogenation of nitroarenes in the presence of sodium borohydride (NaBH₄). It has been found that the reduction mechanism follows the classical Langmuir-Hinshelwood (LH) model at the surface of embedded CDs inside the NDs via the generation of reactive surface hydroxyl groups. These NDs show excellent recyclability without any compromise on reaction kinetics and conversion yield. Importantly, spatiotemporal control over the hydrogenation reaction has been achieved using two mixed populations of coacervates. Moreover, efficient visible light-induced photoredox conversion of ferricyanide to ferrocyanide and artificial peroxidase-like activity have also been demonstrated inside these catalytic NDs. Our findings indicate that the individual polymer-bound CD inside the NDs acts as the catalytic center for both the redox and photocatalytic reactions. The present study highlights the unprecedented catalytic activity of the metal-free CD-based coacervate NDs and paves the way for next-generation catalytic nanoreactors for a wide range of chemical and enzymatic transformations.