Nitrogen Configuration Effects on Charge Carrier Dynamics in CsPbBr3/Carbon Dots S-Scheme Heterojunction for Photocatalytic CO2 Reduction.

Nitrogen-doped carbon dots (NCDs) featuring primary pyrrolic N and pyridinic N dominated configurations were prepared using hydrothermal (H-NCDs) and microwave (M-NCDs) methods, respectively. These H-NCDs and M-NCDs were subsequently applied to decorate CsPbBr3 nanocrystals (CPB NCs) individually, using a ligand-assisted reprecipitation process. Both CPB/M-NCDs and CPB/H-NCDs nanoheterostructures (NHSs) exhibited S-scheme charge transfer behavior, which enhanced their performance in photocatalytic CO2 reduction and selectivity of CO2-to-CH4 conversion, compared to pristine CPB NCs. The presence of pyrrolic N configuration at the heterojunction of CPB/H-NCDs facilitated efficient S-scheme charge transfer, leading to a remarkable 43-fold increase in photoactivity. In contrast, CPB/M-NCDs showed only a modest 3-fold enhancement in photoactivity, which was attributed to electron trapping by pyridinic N at the heterojunction. The study offers crucial insights into charge carrier dynamics within perovskite/carbon NHSs at the molecular level to advance the understanding of solar fuel generation.

Effect of Lattice Disorder on Exciton Dynamics in Copper-Doped InP/ZnSexS1-x Core/Shell Quantum Dots.

InP/ZnSexS1-x core/shell quantum dots (QDs) with varying Cu concentrations were synthesized by a one-pot hot-injection method. X-ray diffraction and high-resolution transmission electron microscopy results indicate that Cu doping did not alter the crystal structure or particle size of the QDs. The optical shifts in UV-visible absorption and photoluminescence (PL) suggest changes in the electronic structure and induction of lattice disorder due to Cu doping. Ultrafast transient absorption spectroscopy (TAS) reveled that a higher Cu-doping level leads to faster charge carrier recombination, likely due to increased nonradiative decay from defect states. Time-resolved PL (TRPL) studies show longer average lifetimes of charge carriers with increased Cu doping. These findings informed the development of a kinetic model to better understand how Cu-induced disorder affects charge carrier dynamics in the QDs, which is important for emerging applications of Cu-doped InP/ZnSexS1-x QDs in optoelectronics.

Charge Carrier Dynamics of CsPbBr3/g-C3N4 Nanoheterostructures in Visible-Light-Driven CO2-to-CO Conversion.

The photon energy-dependent selectivity of photocatalytic CO2-to-CO conversion by CsPbBr3 nanocrystals (NCs) and CsPbBr3/g-C3N4 nanoheterostructures (NHSs) was demonstrated for the first time. The surficial capping ligands of CsPbBr3 NCs would adsorb CO2, resulting in the carboxyl intermediate to process the CO2-to-CO conversion via carbene pathways. The type-II energy band structure at the heterojunction of CsPbBr3/g-C3N4 NHSs would separate the charge carriers, promoting the efficiency in photocatalytic CO2-to-CO conversion. The electron consumption rate of CO2-to-CO conversion for CsPbBr3/g-C3N4 NHSs was found to intensively depend on the rate constant of interfacial hole transfer from CsPbBr3 to g-C3N4. An in situ transient absorption spectroscopy investigation revealed that the half-life time of photoexcited electrons in optimized CsPbBr3/g-C3N4 NHS was extended two times more than that in the CsPbBr3 NCs, resulting in the higher probability of charge carriers to carry out the CO2-to-CO conversion. The current work presents important and novel insights of semiconductor NHSs for solar energy-driven CO2 conversion.

Mechanisms of biochar enhanced Cu2O photocatalysts in the visible-light photodegradation of sulfamethoxazole.

Cu2O nanoparticles are decorated with biochars derived from spent coffee grounds (denoted as Cu2O/SCG) and applied as visible-light-active photocatalysts in the sulfamethoxazole (SMX) degradation. The physicochemical properties of Cu2O/SCG are identified by various spectral analysis, electrochemical and photochemical techniques. As a result, the Cu2O/SCG exhibits the higher removal efficiency of SMX than the pristine Cu2O under visible light irradiation. We can observe that Cu2O could be incorporated onto the SCG biochars with rich oxygen vacancies/adsorbed hydroxyl groups. In addition, the Cu2O/SCG has the lower charge transfer resistance, faster interfacial electron transfer kinetics, decreased recombination of charge carriers and superior absorbance of visible light. The construction of band diagrams for Cu2O/SCG and pristine Cu2O via UV-vis spectra and Mott-Schottky plots suggest that the band energy shifts and higher carrier density of Cu2O/SCG may be responsible for the photocatalytic activity enhancements. From the radical scavenger experiments and electron paramagnetic resonance spectra, the aforementioned energy shifts could decrease the energy requirement of transferring photoinduced electrons to the potential for the formation of active superoxide radicals (·O2-) via one and two-electron reduction routes in the photocatalytic reaction. A proposed degradation pathway shows that ·O2- and h+ are two main active species which can efficiently degrade SMX into reaction intermediates by oxidation, hydroxylation, and ring opening. This research demonstrates the alternative replacement of conventional carbon materials for the preparation of biochar-assisted Cu2O photocatalysts which are applied in the environmental decontamination by using solar energy.

Crystal Facet Dependent Energy Band Structures of Polyhedral Cu2O Nanocrystals and Their Application in Solar Fuel Production.

We demonstrated a facile hydrothermal method to synthesize the (100)-, (110)- and (111)-oriented Cu2O nanocrystals (NCs) by controlling the concentration of the incorporated anions (CO32- and SO32-). The crystal facet dependent activity of the orientation controlled Cu2O NCs in the rhodamine B (RhB) photodegradation and photocatalytic hydrogen (H2) evolution was found to follow the trend: (111) > (110) > (100). The mechanism was investigated by characterizing the optical property, energy band structure, interfacial charge carrier dynamics and reducing ability. The results indicated that the (111)-oriented Cu2O NCs exhibit the higher conduction band (CB) potential as compared with the (110)-oriented and (100)-oriented Cu2O NCs, which resulted in the largest driving force of interfacial electron transfer for (111)-oriented Cu2O NCs to carry out solar fuel generation. The current study offers an easy strategy for crystal facet engineering of semiconductors and provides important physical insights into their electronic properties for the desired solar energy conversions.

Two-Step Process of a Crystal Facet-Modulated BiVO4 Photoanode for Efficiency Improvement in Photoelectrochemical Hydrogen Evolution.

The photoactivity of nanoporous bismuth vanadate (BiVO4, BVO) photoanodes that were fabricated by a two-step process (electrodeposition and then thermal conversion) in photoelectrochemical (PEC) hydrogen (H2) evolution can be enhanced about 1.44-fold by improving the constitutive ratio of (111̅), (061), and (242̅) crystal facets. The PEC characterization was carried out to investigate the factors altering the performance, which revealed that the crystal facet modulation could improve the photoactivity of the BVO photoanodes. In addition, the orientation-controlled BVO thin-film electrodes are introduced as evidence that the present crystal facet modulation is the positive effect for BVO photoanodes in PEC. The investigation of energy band structures and interfacial charge carrier dynamics of the BVO photoanodes reveals that the crystal facet modulation could result in a shorter lifetime of charge carrier recombination and larger band bending at the interface between BVO and electrolytes. This outcome could improve the charge separation and charge transfer efficiencies of BVO photoanodes, promoting the efficiency of PEC H2 evolution. Moreover, this crystal facet modulation can combine with co-catalyst decoration to further improve the solar-to-hydrogen efficiency of BVO photoanodes in PEC. This study presents a potential strategy to promote the PEC activity by crystal facet modulation and important insights into the interfacial charge transfer properties of semiconductor photoelectrodes for the application in solar fuel generation.

Effects of Interfacial Oxidative Layer Removal on Charge Carrier Recombination Dynamics in InP/ZnSexS1-x Core/Shell Quantum Dots.

Red-light-emitting InP/ZnSexS1-x core/shell quantum dots (QDs) were prepared by one-pot synthesis with optimal hydrogen fluoride (HF) treatment. Most of the surficial oxidative species could be removed, and the dangling bonds would be passivated by Zn ions for the InP cores during HF treatment, which would be beneficial to the subsequent ZnSexS1-x shell coating. Three-dimensional time-resolved photoluminescence spectra of the QD samples were analyzed by singular value decomposition global fitting to determine the radiative and nonradiative lifetimes of charge carriers. A proposed model illustrated that the charge carriers in the InP/ZnSexS1-x QDs with interfacial oxidative layer removal would evidently recombine through radiative pathways, mainly from the conduction band to the valence band (lifetime, 33 ns) and partially from the trap states (lifetime, 150 ns). This work offers the important physical insight into the charge carrier dynamics of low-toxicity QDs which have the desired optical properties for optoelectronic applications.

Synergistic Effects of Surface Passivation and Charge Separation to Improve Photo-electrochemical Performance of BiOI Nanoflakes by Au Nanoparticle Decoration.

We demonstrate that the photoactivity of bismuth oxyiodide (BiOI) nanoflake (NF) photocathodes in photo-electrochemical (PEC) water splitting can be significantly enhanced by about 24-fold by thermal calcination under an air atmosphere and then surficial decoration of Au nanoparticles (NPs). To understand the key factors affecting the PEC efficiency in Au NP-decorated BiOI NF photoelectrodes, incident photon-to-current conversion efficiency, electrochemical impedance spectroscopy, photovoltage, and electrochemically active surface area measurements were performed. The analytic results presented that thermal calcining could produce mesopores, increasing active sites on the surface of BiOI NFs. In addition, the synergistic effects of surface-state passivation and charge separation were observed for the surficial Au NP decoration on BiOI NFs. Transient absorption spectroscopy coupled with PEC measurements confirmed that the lifetime of photogenerated electrons on the conduction band of BiOI NFs can be prolonged by Au NP decoration, resulting in higher probability to carry out water reduction. The current investigation presents important insights into the mechanism of charge carrier dynamics in metal-semiconductor nano-heterostructures, which is contributive to develop photoelectrode materials in solar fuel production.

Aspect Ratio-Dependent Charge Carrier Dynamics in Matchstick-like Ag2S-ZnS Nanorods for Solar Hydrogen Generation.

Matchstick-like Ag2S-ZnS nanorods (NRs) with a tunable aspect ratio (AR) were synthesized using one-pot thermal decomposition. The ultraviolet photoelectron spectra and time-resolved photoluminescence spectra of the Ag2S-ZnS NRs were collected to study their electronic band structures and charge carrier dynamics. The energy difference (ΔE) at the interface between the ZnS stem and Ag2S tip was altered as the AR of Ag2S-ZnS NRs increased from 11.9 to 18.4, resulting in an enlarged driving force for the delocalized electrons along the conduction band of ZnS being injected into that of Ag2S. The interfacial electron transfer rate constant (ket) from ZnS to Ag2S could be enhanced by ∼2 orders of magnitude from 5.27 × 106 to 3.24 × 108 s-1, leading to a significant improvement in the efficiency of solar hydrogen generation. This investigation provides new physical insights into the manipulation of charge carrier dynamics by means of AR adjustment in semiconductor nanoheterostructures for photoelectric conversions.

New Insights into the Electron-Collection Efficiency Improvement of CdS-Sensitized TiO2 Nanorod Photoelectrodes by Interfacial Seed-Layer Mediation.

Titanium dioxide (TiO2) nanorods (NRs) are widely used as photoanodes in photoelectrochemical (PEC) solar fuel production because of their remarkable photoactivity and stability. In addition, TiO2 NR electrode materials can be decorated with active CdS quantum dots (QDs) to expand the sunlight photon capture. The overall photoelectric conversion efficiency for TiO2 NR or QD-sensitized TiO2 NR electrode materials in PEC is typically dominated by their interfacial electron transfer (ET) properties. To understand the key factors affecting the ET, the anatase TiO2 seed layer was added into the interface between the rutile TiO2 NRs and fluorine-doped tin oxide (FTO) substrate. This seed layer enhanced the photocatalytic performance of both the TiO2 NR and CdS QD-sensitized TiO2 NR photoanodes in PEC. Time-resolved photoluminescence spectroscopy and PEC analyses, including Mott-Schottky, electrochemical impedance spectroscopy, and photovoltage ( Vph) measurements, were used to study the charge-carrier dynamics at the interfaces between the FTO, TiO2, and CdS QD. Analysis of the results showed that band alignment at the anatase/rutile junction between the TiO2 and FTO promoted electron-collection efficiency ( eEC) at the FTO/TiO2 interface and ET rate constant ( kET) at the TiO2/CdS QD interface. Furthermore, 34% enhancement of the efficiency in hydrogen (H2) generation demonstrated the potential of the TiO2 seed-layer-mediated TiO2/CdS QD NR photoanode in the application of PEC solar fuel production. The current work represents new insights into the mechanism of ET in TiO2 and TiO2/CdS QD NR, which is very useful for the development of photoelectrode materials in solar energy conversions.

Towards understanding the unusual photoluminescence intensity variation of ultrasmall colloidal PbS quantum dots with the formation of a thin CdS shell.

In this study, we report anomalous size-dependent photoluminescence (PL) intensity variation of PbS quantum dots (QDs) with the formation of a thin CdS shell via a microwave-assisted cation exchange approach. Thin shell formation has been established as an effective strategy for increasing the PL of QDs. Nonetheless, herein we observed an unusual PL decrease in ultrasmall QDs upon shell formation. We attempted to understand this abnormal phenomenon from the perspective of trap density variation and the probability of electrons and holes reaching surface defects. To this end, the quantum yield (QY) and PL lifetime (on the ns-µs time scales) of pristine PbS QDs and PbS/CdS core/shell QDs were measured and the radiative and non-radiative recombination rates were derived and compared. Moreover, transient absorption (TA) analysis (on the fs-ns time scale) was performed to better understand exciton dynamics at early times that lead to and affect longer time dynamics and optical properties such as PL. These experimental results, in conjunction with theoretical calculations of electron and hole wave functions, provide a complete picture of the photophysics governing the core/shell system. A model was proposed to explain the size-dependent optical and dynamic properties observed.

Graphene quantum dots conjugated with polymers for two-photon properties under two-photon excitation.

Few studies have investigated the two-photon properties of graphene quantum dots (GQDs) and GQD-conjugated polymers. The results of the present study revealed that conjugated polymers containing nitrogen and sulfur atoms caused higher quantum confinement of emissive energy to be trapped on the surface of nanomaterials, resulting in a high-photoluminescence quantum yield and notable two-photon properties. Additionally, the nanomaterials generated no reactive oxygen species-dependent oxidative stress on cells and served as promising two-photon contrast probes.

Two-Photon Photoexcited Photodynamic Therapy and Contrast Agent with Antimicrobial Graphene Quantum Dots.

A graphene quantum dot (GQD) used as the photosensitizer with high two-photon absorption in the near-infrared region, a large absolute cross section of two-photon excitation (TPE), strong two-photon luminescence, and impressive two-photon stability could be used for dual modality two-photon photodynamic therapy (PDT) and two-photon bioimaging with an ultrashot pulse laser (or defined as TPE). In this study, a GQD efficiently generated reactive oxygen species coupled with TPE, which highly increased the effective PDT ability of both Gram-positive and -negative bacteria, with ultralow energy and an extremely short photoexcitation time generated by TPE. Because of its two-photon properties, a GQD could serve as a promising two-photon contrast agent for observing specimens in depth in three-dimensional biological environments while simultaneously proceeding with PDT action to eliminate bacteria, particularly in multidrug-resistant (MDR) strains. This procedure would provide an efficient alternative approach to easily cope with MDR bacteria.

Organolead Halide Perovskite Nanocrystals: Branched Capping Ligands Control Crystal Size and Stability.

CH3 NH3 PbBr3 perovskite nanocrystals (PNCs) of different sizes (ca. 2.5-100 nm) with high photoluminescence (PL) quantum yield (QY; ca. 15-55 %) and product yield have been synthesized using the branched molecules, APTES and NH2 -POSS, as capping ligands. These ligands are sterically hindered, resulting in a uniform size of PNCs. The different capping effects resulting from branched versus straight-chain capping ligands were compared and a possible mechanism proposed to explain the dissolution-precipitation process, which affects the growth and aggregation of PNCs, and thereby their overall stability. Unlike conventional PNCs capped with straight-chain ligands, APTES-capped PNCs show high stability in protic solvents as a result of the strong steric hindrance and propensity for hydrolysis of APTES, which prevent such molecules from reaching and reacting with the core of PNCs.

Ultrafast Exciton Dynamics in InGaN/GaN and Rh/Cr2O3 Nanoparticle-Decorated InGaN/GaN Nanowires.

Ultrafast exciton and charge-carrier dynamics in InGaN/GaN nanowires (NWs) with and without Rh/Cr2O3 nanoparticle (NP) decoration have been investigated using femtosecond transient absorption (TA) techniques with excitation at 415 nm and white-light probe (450-700 nm). By comparing the TA profiles between InGaN/GaN and InGaN/GaN-Rh/Cr2O3 NWs, an additional decay component on the medium time scale (∼50 ps) was identified with Rh/Cr2O3 decoration, which is attributed to interfacial charge transfer from InGaN/GaN NWs to Rh/Cr2O3 NPs, desired for light energy conversion applications. This is consistent with reduced photoluminescence (PL) of the NWs by the Rh/Cr2O3 NPs. A kinetic model was developed to explain the TA results and gain further insight into the exciton and charge-carrier dynamics.

Studies on the annealing and antibacterial properties of the silver-embedded aluminum/silica nanospheres.

Substantial silver-embedded aluminum/silica nanospheres with uniform diameter and morphology were successfully synthesized by sol-gel technique. After various annealing temperatures, the surface mechanisms of each sample were analyzed using scanning electron microscope, transmission electron microscope, and X-ray photoelectron spectroscopy. The chemical durability examinations and antibacterial tests of each sample were also carried out for the confirmation of its practical usage. Based on the result of the above analyses, the silver-embedded aluminum/silica nanospheres are eligible for fabricating antibacterial utensils.

A facile green antisolvent approach to Cu2+-doped ZnO nanocrystals with visible-light-responsive photoactivities.

An environmentally benign antisolvent method has been developed to prepare Cu(2+)-doped ZnO nanocrystals with controllable dopant concentrations. A room temperature ionic liquid, known as a deep eutectic solvent (DES), was used as the solvent to dissolve ZnO powders. Upon the introduction of the ZnO-containing DES into a bad solvent which shows no solvation to ZnO, ZnO was precipitated and grown due to the dramatic decrease of solubility. By adding Cu(2+) ions to the bad solvent, the growth of ZnO from the antisolvent process was accompanied by Cu(2+) introduction, resulting in the formation of Cu(2+)-doped ZnO nanocrystals. The as-prepared Cu(2+)-doped ZnO showed an additional absorption band in the visible range (400-800 nm), which conduced to an improvement in the overall photon harvesting efficiency. Time-resolved photoluminescence spectra, together with the photovoltage information, suggested that the doped Cu(2+) may otherwise trap photoexcited electrons during the charge transfer process, inevitably depressing the photoconversion efficiency. The photoactivity of Cu(2+)-doped ZnO nanocrystals for photoelectrochemical water oxidation was effectively enhanced in the visible region, which achieved the highest at 2.0 at% of Cu(2+). A further increase in the Cu(2+) concentration however led to a decrease in the photocatalytic performance, which was ascribed to the significant carrier trapping caused by the increased states given by excessive Cu(2+). The photocurrent action spectra illustrated that the enhanced photoactivity of the Cu(2+)-doped ZnO nanocrystals was mainly due to the improved visible photon harvesting achieved by Cu(2+) doping. These results may facilitate the use of transition metal ion-doped ZnO in other photoconversion applications, such as ZnO based dye-sensitized solar cells and magnetism-assisted photocatalytic systems.

Multicolored Cd(1-x)Zn(x)Se quantum dots with type-I core/shell structure: single-step synthesis and their use as light emitting diodes.

We developed a single-step hot-injection process to synthesize Cd1-xZnxSe quantum dots (QDs) with tunable emission wavelengths. The multiple emission colors of the Cd1-xZnxSe QDs resulted from the variation in their compositions (x value) with the reaction time. Because of the higher reactivity of the Cd precursor, QDs whose composition was rich in CdSe were generated at the beginning of the reaction. As the reaction proceeded, the later-formed ZnSe shell was simultaneously alloyed with the core, giving rise to a progressive alloying treatment for the grown QDs. During the reaction period, the emission color of the Cd1-xZnxSe QDs shifted from red to orange, to yellow, to green and finally to blue. A light emitting diode (LED) composed of multilayers of ITO/poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate)/poly(3-hexylthiophene) blended with Cd1-xZnxSe QDs/Al was fabricated to test the electroluminescence (EL) properties of the QDs. The EL results show high color purity for the emission from LED devices containing Cd1-xZnxSe QDs, revealing that the as-synthesized QDs can be easily processed and integrated into a light-emitting device without using a complicated procedure. The findings from the present work also demonstrate the advantage of using the current single-step synthetic approach to obtain a batch of Cd1-xZnxSe QDs that may emit different colors in prototype LEDs.

Au/ZnS core/shell nanocrystals as an efficient anode photocatalyst in direct methanol fuel cells.

Au/ZnS core/shell nanocrystals with controllable shell thicknesses were synthesized using a cysteine-assisted hydrothermal method. Incorporating Au/ZnS nanocrystals into the traditional Pt-catalyzed half-cell reaction led to a 43.3% increase in methanol oxidation current under light illumination, demonstrating their promising potential for metal/semiconductor hybrid nanocrystals as the anode photocatalyst in direct methanol fuel cells.

Au nanostructure-decorated TiO2 nanowires exhibiting photoactivity across entire UV-visible region for photoelectrochemical water splitting.

Here we demonstrate that the photoactivity of Au-decorated TiO2 electrodes for photoelectrochemical water oxidation can be effectively enhanced in the entire UV-visible region from 300 to 800 nm by manipulating the shape of the decorated Au nanostructures. The samples were prepared by carefully depositing Au nanoparticles (NPs), Au nanorods (NRs), and a mixture of Au NPs and NRs on the surface of TiO2 nanowire arrays. As compared with bare TiO2, Au NP-decorated TiO2 nanowire electrodes exhibited significantly enhanced photoactivity in both the UV and visible regions. For Au NR-decorated TiO2 electrodes, the photoactivity enhancement was, however, observed in the visible region only, with the largest photocurrent generation achieved at 710 nm. Significantly, TiO2 nanowires deposited with a mixture of Au NPs and NRs showed enhanced photoactivity in the entire UV-visible region. Monochromatic incident photon-to-electron conversion efficiency measurements indicated that excitation of surface plasmon resonance of Au is responsible for the enhanced photoactivity of Au nanostructure-decorated TiO2 nanowires. Photovoltage experiment showed that the enhanced photoactivity of Au NP-decorated TiO2 in the UV region was attributable to the effective surface passivation of Au NPs. Furthermore, 3D finite-difference time domain simulation was performed to investigate the electrical field amplification at the interface between Au nanostructures and TiO2 upon SPR excitation. The results suggested that the enhanced photoactivity of Au NP-decorated TiO2 in the UV region was partially due to the increased optical absorption of TiO2 associated with SPR electrical field amplification. The current study could provide a new paradigm for designing plasmonic metal/semiconductor composite systems to effectively harvest the entire UV-visible light for solar fuel production.