Conduction Band Electrons Research Articles

Exploring the Modelling of Gold Anisotropic Nanoparticles’ Optical Properties as a Promising Tool for Detection Systems Design

Gold nanoparticles have been a central topic in the last few decades due to their excellent optical properties that can be exploited in many applications, including food analysis, materials science, and biomedicine. The basis of these unique optical properties is the phenomenon known as localized surface plasmon (LSP), which relays in the collective oscillation of the conduction band electrons in the nanoparticle when excited by electromagnetic radiation. The optical properties of the nanoparticles are critical for selecting the best nanomaterials for each application, a key factor for optimum performance, and can be tuned due to their dependence on the geometry and size of the nanoparticles, as well as the polarization of the light beam. Here, we conducted simulations to study the tunable optical properties and local electric field distribution of three types of gold nanoparticles, cubes (AuNC), boxes (AuNB), and triangular prisms (AuNT), which have relatively simple synthetic routes. Finally, we compared these results with experimental data and described possible synthetic routes to discuss the positive and negative aspects of using each type of nanoparticle for potential applications.

The role of conduction band electrons in promoting O2 adsorption to silicon interfaces

Resonance enhanced Second Harmonic Generation (SHG) was employed to assess if conduction band electrons in silicon (Si) will promote molecular adsorption of ambient species and how such adsorption depends on temperature. Experiments were performed with three types of Si (n-doped or n-Si, p-doped or p-Si, and undoped Si) at temperatures between 18 and 260 °C and under atmospheres of dry N2 and dry (cylinder) air. All Si types were covered with a 2–4 nm thick native oxide layer. Under N2, all Si types behave similarly, with SHG intensity [I(2ω)] diminishing with increasing temperature. This effect was reversible and attributed to electron–phonon scattering. In the presence of O2, I(2ω) from n-Si at room temperature is enhanced significantly. Neither p-doped Silicon (p-Si) nor undoped Si show similar effects at room temperature, with I(2ω) being independent of gas phase composition. At temperatures ≥175 °C, all three Si types behaved similarly with no dependence on atmospheric O2 content. Varying the amount of O2 above n-Si at room temperature and measuring I(2ω) suggested that O2 adsorption to n-Si could be described with a Langmuir isotherm and an adsorption energy of −0.13 ± 0.05 eV. Increasing n-Si’s oxide thickness (to 600 nm) rendered the substrate insensitive to ambient gas phase composition. Taken together, these findings support a description of Si’s surface electronic structure that is controlled by n-Si conduction band electrons backbonding into the π* orbitals of adjacent O2 and imply that these conduction band electrons can affect adsorption despite the presence of a native oxide film.

Construction of Z-type heterojunction by depositing CuO nanoparticles on NiO nanosheets for visible-light-enhanced catalytic ammonia borane methanolysis

The development of effective and cost-efficient catalysts for the hydrogen production through methanolysis of ammonia borane (AB) is a hot topic in the field of energy catalysis. In this study, a simple method has been developed to construct a low-cost Z-type heterojunction by depositing CuO nanoparticles on the surface of NiO nanosheets. The as-prepared CuO/NiO composites exhibit prominent activity towards AB methanolysis. The hydrogen generation rate (HGR) is 3.00 Lhydrogen min−1 gcat.−1 when the optimal CuO/NiO sample functions as a catalyst, which is increased to 3.93 Lhydrogen min−1 gcat.−1 with the assistance of visible light illumination. The enhancement of catalytic activity are attributed to the variation of the electronic structure of the catalysts. With the formation of Z-type heterojunction, the valence band electrons of CuO and NiO are excited and transfer to their respective conduction bands under light illumination. Subsequently, the conduction band electrons of CuO recombine with the valence band holes of NiO, leading to an accumulation of electrons at the conduction band of NiO. Consequently, the electron density of the Ni sites is increased, which enhances AB methanolysis. This study can provide new insights for the design and synthesis of cost-effective and highly efficient direct Z-type heterojunction catalysts for hydrogen production through AB methanolysis. Moreover, this study is also helpful for the understanding of how visible light enhances AB methanolysis.

Origin of charges in bulk Si:HfO2 FeFET probed by nanosecond polarization measurements

FeFET technology offers the potential for fast, energy-efficient, low-cost, and high-capacity non-volatile memory and neuromorphic devices. However, charge trapping significantly affects device operation, leading to issues like read-after-write delay and limited endurance. Therefore, a detailed understanding of charge trapping, charge origin and its role in polarization switching is crucial. In this study, we uncover the spectral energy origin of polarization charges in Si:HfO2 N-FeFET by probing electron (conduction band) and hole (valence band) currents separately during polarization-voltage (P–V) measurements. We utilize a fast (∼20 ns) and modified positive-up-negative-down (PUND) technique, where bulk, source, and drain currents of the FeFET are measured separately. The nanosecond timescale of the measurement results in measurable currents in FeFETs having dimensions of a few μm. This charge separation shows that program (PRG, VGS > 0) charge originates from the conduction band, whereas erase (ERS, VGS < 0) originates from the valence band of the Si. Moreover, the polarization curve (P–V) of a cycled device (following 5000 PRG/ERS pulses) shows measurable hysteresis even though the transfer curve of the same device shows that the memory window in the threshold voltage vanishes. Therefore, the FeFET polarization state can be read without delay after write operation by the fast PUND measurement, both for pristine and cycled FeFETs.

Exchange-enhanced spin-orbit splitting and its density dependence for electrons in monolayer transition metal dichalcogenides

We show that spin-orbit splitting (SOS) in monolayers of semiconducting transition metal dichalcogenides (TMDs) is substantially enhanced by an electron-electron interaction. This effect, similar to the exchange enhancement of the electron g factor, is most pronounced for conduction band electrons (in particular, in MoS2), and it has a nonmonotonic dependence on the carrier sheet density n. That is, the SOS enhancement is peaked at the onset of filling of the higher-energy spin-split band by electrons, n*, which also separates the regimes of slow (at n&lt;n*) and fast (for n&gt;n*) spin and valley relaxation of charge carriers. Moreover, this density itself is determined by the enhanced SOS value, making the account of exchange renormalization important for the analysis of the spintronic performance of field-effect transistors based on two-dimensional TMDs. Published by the American Physical Society 2024

Pressure-controlled luminescence in fast-response barium fluoride crystals

Cross-luminescence (CL) in a barium fluoride (BaF2) scintillator arising from the recombination of a valence band electron and a core band hole results in a fast picosecond decay time. However, the CL emission wavelength in the vacuum ultraviolet region is difficult to detect, and intrinsically intense and slow nanosecond self-trapped exciton (STE) luminescence occurs. Herein, we report a redshift in the CL emission wavelength with high-pressure application. The wavelength of the CL emission shifted from 221 nm to 240 nm when 5.0 GPa was applied via a sapphire anvil cell. Increasing the pressure decreases the core-valence bandgap due to the downward expansion of the valence band, resulting in a decrease in the valence band minimum. The onset of a phase transition from a cubic crystal structure to an orthorhombic crystal structure at 3.7 GPa inhibited the recombination of conduction band electrons and self-trapped holes, leading to the disappearance of the STE emission. Manipulating the band structure of BaF2 by high-pressure application enables control of its luminescence emission, providing a pathway toward solving the problems inherent in this leading fast-response scintillator.

Charge carrier absorption in n-type Sb2Se3

The optical and electrical properties of n-type chlorine-doped Sb2Se3 single crystals, with free carrier concentrations above 1016 cm−3 at room temperature, have been studied. The experiments reveal a strongly polarized temperature-dependent long-wavelength infrared absorption attributable to conduction band electrons within the material. For wavelengths between 1.6 and 6 μm, the room temperature absorption varies as λ2.5±0.3, suggesting that longitudinal optical mode scattering is the dominant electron scattering mechanism. The results are most consistent with the hypothesis that electron transport in Sb2Se3 is band-like and not intrinsically limited by small-polaron self-trapping.

A room-temperature ultrafast carrier dynamical study and thickness-dependent investigation of WTe2 thin films on a flexible PET substrate

In the current era of increasing demand for optoelectronic-based devices with ultra-rapid response, it is important to understand the processes associated with the relaxation dynamics of hot carriers and transient electrical properties of WTe2 material under photoexcitation of charge carriers. In this work, using femtosecond laser pump–probe spectroscopy at room temperature we performed the transient absorption measurement on sputtered deposited WTe2 thin films having four different thicknesses to study dynamics associated with the relaxation of their hot carriers. The relaxation dynamics of photoexcited charge carriers undergo three exponential decay components associated with electron–phonon thermalization in the conduction band and phonon-assisted electron–hole recombination between the electron and hole pocket. The thickness-dependent investigation of WTe2 thin films reveals that the electron–hole recombination process is more prominent in thicker films than in thinner films, supporting previously published theoretical and experimental conclusions. The Ultrafast study of WTe2 thin films suggests that it is a suitable material for future ultrafast optoelectronic-based device applications.

Indirect to direct band gap engineering of cubic bromide perovskite AlMgBr3 under pressure: First-principles calculations for enhanced optoelectronic applications

Through first principles, the structural, electronic, optical, mechanical, and thermodynamic properties of AlMgBr3 at different pressures are investigated for the first time. At 0 GPa, cubic AlMgBr3 is an indirect band gap insulator (Eg = 3.958 eV). Interestingly, its band gap type changes from an indirect band gap to a direct band gap at 5 GPa, making it a more favorable material for optoelectronic device applications. In addition, as the pressure increases, the conduction band electrons move downward, resulting in a reduction of the band gap, and the band gap transition from the ultraviolet to visible region. The optical properties show that AlMgBr3 has a high dielectric constant, refractive index, and conductivity, which is favorable for its application in solar cells, waveguides and other optoelectronic devices. The phase transition point of AlMgBr3 was determined to be 39.6 GPa based on the mechanical stability conditions, and the mechanical properties revealed the ductility, anisotropy and machinability of AlMgBr3. By using the quasi-harmonic Debye model, the effect of temperature and pressure on the bulk modulus, coefficient of thermal expansion, isobaric heat capacity, Debye temperature, and entropy were also calculated. It is worth mentioning that the electronic, optical, mechanical and thermodynamic properties of AlMgBr3 have been significantly enhanced under pressure.

(Invited) Luminescent InP Quantum Dots: They're Not Just Like CdSe

Time-resolved photoluminescence (PL) and transient absorption (TA) spectroscopy have been used to elucidate the electron and hole dynamics in high quality InP/ZnSe/ZnS quantum dots (QDs) at room temperature. These dynamics depend critically on the overall In:P ratio. Stoichiometric QDs (those having an In:P ratio close to unity), have PL quantum yields of > 90% and exhibit dynamics that are very simple: both electron and hole relaxation to the bandedge is fast compared to the ~ 40 ps time-correlated photon-counting instrument response and the PL follows a single exponential decay having a time constant of about 26 ns. However, QDs having an In:P ratio greater than about 1.1 maintain PL quantum yields > 90%, but show much more complicated PL kinetics. In the non-stoichiometric QDs, a significant fraction of the PL exhibits a slower risetime. The PL rises over a range of timescales varying from close to the instrument response to approximately 2.0 ns. The time constant of the slowest component increases with increasing InP core size and the fraction of the PL that has the slow risetime increases with excess indium in the particle. The slow risetime is absent in the TA results, indicating that the slow dynamics may be assigned to relaxation in the valence band. The risetime increases with the thickness of the ZnSe shell, varying from < 40 ps for the thinnest (1.1 nm) to about 2.0 ns for the thickest (2.7 nm) ZnSe shells. In addition to the slow risetime, the QDs having excess indium show a significant long-lived (>100 ns) decay component. The magnitude and time constant of this slow decay also increases with ZnSe shell thickness.These results suggest that holes are transiently trapped at sites associated with indium dopants in the ZnSe shell. The trapped hole has negligible overlap with the conduction band electron, making this an optically dark state. The holes are in an equilibrium between the shell traps and the valence band, giving rise to the delayed emission. Several possible assignments of the trapping species can be considered, and a substitutional indium adjacent to a zinc vacancy, In3+/VZn 2-, is the most likely. This assignment is consistent with the observation that trapping occurs only when the QD has excess indium and is supported by experiments showing that the addition of zinc oleate or acetate decreases the extent of trapping, presumably by filling some of the vacancy traps. Further experiments show that addition of alkyl carboxylic acids causes increased trapping, presumably by the formation of zinc oleate, thereby creating additional zinc vacancies. Chloride ion is a common impurity in the synthesis of InP/ZnSe/ZnS QDs and the possibility of In3+/Cl- impurities acting as the hole trap can be also considered. However, additional experimental data on InP/ZnSe/ZnS QDs synthesized in the absence of chloride show the same PL risetimes and delayed emission, eliminating this possibility. Density functional theory calculations further corroborate assignment of the trapping species to In3+/VZn 2-. These calculations show that either a single In2+ ion or an In2+-In3+ dimer is much too easily oxidized to form the reversible traps observed experimentally, while In3+ is far too difficult to oxidize. However, a zinc vacancy adjacent to a substitutional indium is calculated to have its highest occupied orbitals about 1 eV above the top of the valence band of bulk ZnSe, in the appropriate energy range to act as reversible traps for quantum confined holes in the InP valence band.

Activation/ Deactivation of Surface States of Heterostructure (Anatese-TiO2/Au) Via Addition of Materials with High Band Gap, Rutile TiO2 (R-TiO2) or Low Band Gap Silicon (Si), and Fabrication Strategy

The addition of high band gap or low band gap material not only modulates the optical absorption cross-section but also affects the surface state reactivity of heterostructure towards the photoelectrochemical (PEC) water splitting. Further, the deposition strategy to assemble the heterostructure also affects the effectiveness of the heterostructure. To explore such issues, we fabricated the two sets of heterostructures containing (A-TiO2, R-TiO2) and (Si, A-TiO2) via the click chemistry approach through two different strategies, i.e., layer-by-layer heterostructure (LLH), and randomly deposited heterostructure. In LLH strategy, only the addition of high band gap material R-TiO2 enhances the reactivity of the surface states. On the other hand, the addition of the low band gap material Silicon (Si) hampers the reactivity of the surface states of the heterostructure deposited via the LLH. Further, by changing the deposition strategy to RDH, the reactivity of surface states diminishes by adding either the high band gap or the low band gap material. These observations were rationalized from the framework of band structure/band-alignment in the heterostructure and also via electrochemical impedance spectroscopy (EIS).In LLH, the addition of the A-TiO2 quenches electrons from the conduction band (CB) of R-TiO2, and the holes of A-TiO2 are transferred to R-TiO2. The holes from R-TiO2 participate in activating the surface states present at the surface. On the other hand, the addition of silicon to LLH, having CB above the CB of A-TiO2, does not allow the electrons to transfer from A-TiO2 to Si; hence, the CB electrons from A-TiO2 transfer to surface states, which enhances the energy level of surface states, making it more negative than the VB of Si. The negative shift of surface states of A-TiO2 makes it the recombination center. Interestingly, changing the fabrication strategy from LLH to RDH makes the nature of surface states from reactive to recombination states due to the interfacing of all materials with the underlying substrates.EIS was utilized to measure and analyze the low-frequency capacitance due to the reactive surface state. The low-frequency capacitance of LLH with A-TiO2 shows the maxima at onset potential, confirming the role of reactive surface states. On the contrary, low-frequency capacitance increases monotonically for LLH with Si addition, showing the effect of recombination surface states. Overall, the study provides pointers on the interplay between band positions and electron-hole separation in heterostructures and how they can be effectively used to enhance surface reactivity in PEC water splitting.

Construction of Ag3PO4/g-C3N4 Z-Scheme Heterojunction Composites with Visible Light Response for Enhanced Photocatalytic Degradation.

Ag3PO4/g-C3N4 photocatalytic composites were synthesized via calcination and hydrothermal synthesis for the degradation of rhodamine B (RhB) in wastewater, and characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance spectroscopy (DRS). The degradation of RhB by Ag3PO4/g-C3N4 composites was investigated to evaluate their photocatalytic performance and cyclic degradation stability. The experimental results showed that the composites demonstrated notable photocatalytic activity and stability during degradation. Their high degradation efficiency is attributed to the Z-scheme transfer mechanism, in which the electrons in the Ag3PO4 conduction band and the holes in the g-C3N4 valence band are annihilated by heterojunction recombination, which greatly limits the recombination of photogenerated electrons and holes in the catalyst and enhances the activity of the composite photocatalyst. In addition, measurements of photocurrent (PC) and electrochemical impedance spectroscopy (EIS) confirmed that the efficient charge separation of photo-generated charges stemmed from strong interactions at the close contact interface. Finally, the mechanism for catalytic enhancement in the composite photocatalysts was proposed based on hole and radical trapping experiments, electron paramagnetic resonance (EPR) analysis, and work function evaluation.

(Invited) Light Emission from Single Plasmonic Nanoparticles

A surface plasmon in a metal nanoparticle is the coherent oscillation of the conduction band electrons leading to both absorption and scattering as well as strong local electromagnetic fields. These fundamental properties have been exploited in many different ways, including surface enhanced spectroscopy and sensing, photothermal cancer therapy, and color display generation. The performance of plasmonic nanoparticles for a desired application not only depends on the particle size and shape, but is tunable through nanoparticle interactions on different length scales that support near- and far-field coupling. Chemical synthesis and assembly of nanostructures are able to tailor plasmonic properties that are, however, typically broadened by ensemble averaging. Single particle spectroscopy together with correlated imaging is capable of removing heterogeneity in size, shape, and assembly geometry. In this talk I will discuss our recent work on understanding the radiative properties of individual plasmonic nanostructures including its time dependence, illustrating that light emission provides a unique window into the dynamics of hot carriers.

(Invited) Photophysics and Photochemistry of Individual Plasmonic Nanostructures

A surface plasmon in a metal nanoparticle is the coherent oscillation of the conduction band electrons leading to both absorption and scattering as well as strong local electromagnetic fields. These fundamental properties have been exploited in many different ways, including surface enhanced spectroscopy and sensing, photocatalysis, photothermal cancer therapy, and color display generation. Chemical synthesis and assembly of nanostructures are able to tailor plasmonic properties that are, however, typically broadened by ensemble averaging. Single particle spectroscopy together with correlated imaging is capable of removing heterogeneity in size, shape, and assembly geometry and furthermore allows one to separate absorption and scattering contributions. In this talk I will discuss our recent work on distinguishing the different contributions that cause plasmon decay as probed by the homogeneous single-particle linewidth.[1-4] In particular, I will focus on plasmon damping due to energy and charge transfer from the metal to its environment with the goal to mechanistically understand how energy conversion from an incident photon to a plasmon and then a localized excitation can be maximized while circumventing fast metal-intrinsic internal conversion that simply leads to heat generation.

(Invited) Disentangling the Nonequilibrium Dynamics of Excitations in 2D Material Interfaces Under Bias: Leveraging Theory, Electrochemistry, and Time-Resolved Spectroscopy

A fundamental challenge in elucidating, controlling, and exploiting nonequilibrium relaxation in condensed phase systems is the ability to find and employ physically transparent models. Through such models, one can develop a compelling assignment and interpretation of the spectral signatures of processes spanning charge and energy transfer, quasiparticle formation, and even chemical reactivity. Two-dimensional materials, such as transition metal dichalcogenides (TMDs), offer one such challenge, especially under applied fields. In this talk, I will describe our recent work combining theory, electrochemical measurements, and ultrafast spectroscopy to demonstrate hot carrier extraction in photoelectrochemical cells composed of monolayer MoS2 on an ITO electrode exposed to electrolyte solution. In addition, I will discuss our critical assessment of the ability of a physically intuitive Hamiltonian, consisting of an infinitely heavy exciton immersed in a fermi sea of conduction band electrons, to capture and offer an interpretation of the spectral features in the linear and transient absorption optical signals of this material under an applied bias. Importantly, we leverage our analysis to identify, physically, how trion formation moves, broadens, and resizes the “A exciton” absorption of our working device. We further unify the interpretation over various sources of such TMD spectral shifts: applied bias, fluence, and (TA) delay time. Our work thus demonstrates hot carrier extraction in 2D photoelectrochemical cells, outlines a path to exploit this phenomenon in semiconductor devices, delineates open questions that are only now becoming possible to address with theory and experiment, and identifies the underlying physical process responsible for previously misidentified spectral features in 2D materials that changed with applied voltage, fluence, and time.

New Insights in Luminescence and Quenching Mechanisms of Ag2S Nanocrystals through Temperature-Dependent Spectroscopy.

Bright near-infrared-emitting Ag2S nanocrystals (NCs) are used for in vivo temperature sensing relying on a reversible variation in intensity and photoluminescence lifetime within the physiological temperature range. Here, to gain insights into the luminescence and quenching mechanisms, we investigated the temperature-dependent luminescence of Ag2S NCs from 300 to 10 K. Interestingly, both emission and lifetime measurements reveal similar and strong thermal quenching from 200 to 300 K, indicating an intrinsic quenching process that limits the photoluminescence quantum yield at room temperature, even for perfectly passivated NCs. The low thermal quenching temperature, broadband emission, and multiexponential microsecond decay behavior suggest the optical transition involves strong lattice relaxation, which is consistent with the recombination of a Ag+-trapped hole with a delocalized conduction band electron. Our findings offer valuable insights for understanding the optical properties of Ag2S NCs and the thermal quenching mechanism underlying their temperature-sensing capabilities.

Photocorrosion suppression of copper sulfide via hybridization with amino-functionalized metal–organic framework for natural sunlight-driven photocatalysis

Natural sunlight-driven photocatalysis has been considered as an energy sustainable strategy for the synthesis of fine chemicals, since natural sunlight is inexhaustible. Copper sulfide (CuS) with broad light absorption holds great promise for natural sunlight utilization, while hole-induced photocorrosion seriously hampers its photoactivity and photostability. Herein, we inhibited the photocorrosion by hybridizing CuS with amino-functionalized metal–organic framework of NH2-MIL-125(Ti) (NM125) to fabricate a Z-scheme heterojunction. Taking advantage of high binding affinity of amino group and copper ion, CuS nanoparticles (NPs) were in situ anchored on NM125 surface via amino group-assisted coordination. Characterization results suggested the diffusion-controlled charge migration in NM125@CuS(x) hybrids, where photoinduced electrons in conduction band (CB) of NM125 transferred to valence band (VB) of CuS, and then combined here. The Z-scheme charge transport could timely consume the photogenerated holes of CuS, and spatially separated the charge carriers. As a result, the obtained NM125@CuS(x) catalysts were highly efficient and stable in natural sunlight-driven Phillips-Ladenburg reaction in air, providing serial benzimidazoles with excellent yields (90–98%). Such natural sunlight-driven photocatalysis system efficiently utilized natural sunlight to synthesize high-value fine chemicals in open air at mild conditions, which opens up a sustainable avenue for the green development of fine chemical engineering.

Prediction of Shockley–Read–Hall lifetimes in strained layer superlattices for mid-wave and long-wave infrared photodetectors

We have calculated carrier nonradiative recombination lifetimes limited by Shockley–Read–Hall (SRH) centers in strained layer superlattices (SLSs) for mid-wave and long-wave infrared applications. The capture rate of an electron (hole) in the SLS's conduction (valence) band by the defect level is dominated by a multi-phonon process, which is orders-of-magnitude more efficient than the radiative process. Long-range polar coupling between electrons and optical phonons can account for the observed SRH lifetimes in a variety of SLSs reported in the literature. The capture rate depends on temperature rather weakly, consistent with experimental observations. The efficient capture is caused by the comparable electronic difference and lattice relaxation energy, Ect∼Sℏω, with S and ℏω being the Huang–Rhys factor and optical photon energy in the SLSs. A weaker polar coupling would give rise to a smaller capture cross section, which, for InAs/InAsSb SLSs, can be achieved by increasing Sb in the alloy region.

Macrocyclic porphyrin photocatalysts without metal chelation: A novel pathway for complete degradation of tough halophenols with longwave visible LED light source

Halophenols are toxic and persistent pollutants in water environments which poses harm to various organisms. Due to their high stability and long residence time, ultraviolet radiation, heavy metals and oxidizing agents have been largely adopted on treating these compounds. However, these treatment methods could pose toxicity or hazardous risks to the marine environment and plant operators. In this study, a water-soluble porphyrin photocatalyst was synthesized and introduced for halophenol treatment using UV-free LED white light. The porphyrin catalyst is a macrocyclic ring consisting of pyrroles linked with methine bridges, the highly conjugated ring provided the superior functionality of visible light absorption. Surprisingly, over 99 % degradation of halophenols and over 90 % dehalogenation have been achieved without metal chelation, even higher than those of transition metal porphyrins with inclusion of Fe3+, Zn2+, Cu2+, Co2+, Ni2+, and Mn2+. Ring-opening reactions were confirmed with the formation of carboxylic acids; dicarboxylic acids like acrylic acid, and malonic acid; while fumaric acid was the main product. Total organic carbon results indicated no CO2 produced during the reaction. Triplet absorbance and scavenger studies also indicated that singlet oxygen and conduction band electrons are the main radical species for halophenol degradation. The 100-fold singlet emission quenching over triplet absorption quenching indicated that the excited electrons tend to be transferred via singlet state. This concept brings along new approaches detoxifying halophenol-related wastewater without UV, metals and other additives, which is more environmentally-friendly and sheds light to the conversion of toxic materials into useful chemical precursors.

Interlinking the Fe doping concentration, optoelectronic properties, and photocatalytic performance of ZnO nanostructures

Doping is one of the effective strategies to modulate the optoelectronic properties and photocatalytic activity of photocatalysts. In this study, the effect of Fe doping (0–10 %) on morphology, optical and electronic properties, and photocatalytic activity of ZnO nanostructures is studied. The X-ray diffraction analysis shows that >5 % Fe doping, ZnFe2O4 is segregated as a secondary phase. The crystalline size decreases from 50.8 nm to 21.4 nm and the micro-strain increases with increasing the Fe concentration. The Fe doping-induced electronic restructuring facilitates visible light absorption through the O 2p → Fe 3d transition and the suppression of charge recombination by efficiently trapping conduction band electrons. Density functional theory (DFT) calculations are employed to unravel the underlying electronic changes induced by Fe doping in ZnO. The formation of shallow donor levels below the conduction band originates from the Fe 3d state. Photoluminescence spectra of pristine and Fe-doped ZnO nanostructures show characteristic emission peaks at approximately 384 nm and 570 nm, indicating the recombination of free excitons and oxygen interstitial defects, respectively. The results of the photocatalytic activity tests confirm that the 1 % Fe-doped ZnO nanostructures can exhibit the highest efficiency compared to the heavily doped ZnO nanostructures. The high efficiency in photocatalytic activity of 1 % Fe-doped ZnO nanostructures is ascribed to the modulated electronic structure and defect density. The adsorption affinity of methylene blue and water molecules to the surfaces of pristine and Fe-doped ZnO is simulated using the Monte-Carlo method. This study emphasizes the importance of controlling the dopant concentration to enhance the photocatalytic activity of various photocatalysts.