Photoexcited Charge Carriers Research Articles

Hierarchical structures of 2D g-C3N4 sheet supported 3D star-shaped flower CuO nanohybrids for improved photoelectrochemical water oxidation and visible light induced photodegradation of antibiotics

The interfacial interaction between different materials is favorable to the photoexcited charge separation, and it can also contribute complete performance to their individual benefits and create for the shortcomings to attain a complementary influence. But, it is still challenging to modify the preferred structure and heterojunction interface of catalyst materials using a facile and effective method. In this report, 2D g-C3N4 sheets supported 3D star-shaped flower like CuO nanohybrid p-n type heterojunction catalysts were prepared using simple facile synthesis technique and examined their catalytic activity and photoelectrochemical properties. The photocatalytic efficacies of TC using pure g-C3N4, CuO and CuO/g-C3N4 were 47.4%, 75.2% and 94.7% within the 30 min of visible light irradiation. The enhanced photodegradation performance of nanohybrids catalyst is due to the formation of heterojunction, subsequent in the photoexcited charge carrier’s active separation the improved ability of visible light absorption and development of p-n heterojunction. Moreover, PEC water oxidation properties were examined and CuO/g-C3N4 nanohybrid electrodes displays improved PEC properties than the pure CuO and g-C3N4. Furthermore, g-C3N4, CuO and CuO/g-C3N4 photoelectrode displays photocurrent density 0.85, 1.60 and 2.30 mA/cm−2, respectively. Therefore, 3D/2D nanohybrids with robust ability of visible light absorption could offer a novel method to create effective facilitating the growth of photoelectrodes for the PEC water oxidation and photodegradation for fast and effective antibiotic removal from the environment.

Sunlight-driven photocatalytic conversion of furfural and its derivatives

Sunlight-driven photocatalytic conversion of furfural and its derivatives enables activation and transformation of the target chemical bonds under very mild conditions using photoexcited charge carriers or photogenerated reactive species.

Manipulation of interfacial charge dynamics for metal-organic frameworks toward advanced photocatalytic applications.

Compared to other known materials, metal-organic frameworks (MOFs) have the highest surface area and the lowest densities; as a result, MOFs are advantageous in numerous technological applications, especially in the area of photocatalysis. Photocatalysis shows tantalizing potential to fulfill global energy demands, reduce greenhouse effects, and resolve environmental contamination problems. To exploit highly active photocatalysts, it is important to determine the fate of photoexcited charge carriers and identify the most decisive charge transfer pathway. Methods to modulate charge dynamics and manipulate carrier behaviors may pave a new avenue for the intelligent design of MOF-based photocatalysts for widespread applications. By summarizing the recent developments in the modulation of interfacial charge dynamics for MOF-based photocatalysts, this minireview can deliver inspiring insights to help researchers harness the merits of MOFs and create versatile photocatalytic systems.

Pattern-illumination time-resolved phase microscopy and its applications for photocatalytic and photovoltaic materials.

Pattern-illumination time-resolved phase microscopy (PI-PM) is a technique used to study the microscopic charge carrier dynamics in photocatalytic and photovoltaic materials. The method involves illuminating a sample with a pump light pattern, which generates charge carriers and they decay subsequently due to trapping, recombination, and transfer processes. The distribution of photo-excited charge carriers is observed through refractive index changes using phase-contrast imaging. In the PI-PM method, the sensitivity of the refractive index change is enhanced by adjusting the focus position, the method takes advantage of photo-excited charge carriers to observe non-radiative processes, such as charge diffusion, trapping in defect/surface states, and interfacial charge transfer of photocatalytic and photovoltaic reactions. The quality of the image sequence is recovered using various informatics calculations. Categorizing and mapping different types of charge carriers based on their response profiles using clustering analysis provides spatial information on charge carrier types and the identification of local sites for efficient and inefficient photo-induced reactions, providing valuable information for the design and optimization of photocatalytic materials such as the cocatalyst effect.

Charge carrier recombination processes, intragap defect states, and photoluminescence mechanisms in stoichiometric and reduced TiO2 brookite nanorods: an interpretation scheme through in situ photoluminescence excitation spectroscopy in controlled environment.

The study of titanium dioxide (TiO2) in the brookite phase is gaining popularity as evidence has shown the efficient photocatalytic performance of this less investigated polymorph. It has been recently reported that defective anisotropic brookite TiO2 nanorods display remarkable substrate-specific reactivity towards alcohol photoreforming, with rates of hydrogen production significantly (18-fold) higher than those exhibited by anatase TiO2 nanoparticles. To elucidate the basic photo-physical mechanisms and peculiarities leading to such an improvement in the photoactive efficiency, we investigated the recombination processes of photoexcited charge carriers in both stoichiometric and reduced brookite nanorods via photoluminescence excitation spectroscopy in controlled environment. Through an investigation procedure employing both supragap and subgap excitation during successive exposure to oxidizing and reducing gaseous agents, we firstly obtained an interpretation scheme describing the main photoluminescence and charge recombination pathways in stoichiometric and reduced brookite, which includes information about the spatial and energetic position of the intragap states involved in photoluminescence mechanisms, and secondly identified a specific photoluminescence enhancement process occurring in only reduced brookite nanorods, which indicates the injection of a conduction band electron during ethanol photo-oxidation. The latter finding may shed light on the empirical evidence about the exceptional reactivity of reduced brookite nanorods toward the photo-oxidation of alcohols and the concomitant efficiency of photocatalytic hydrogen generation.

Efficient photo-driven ion pump through slightly reduced vertical graphene oxide membranes.

Solar energy can be harvested using biological light-driven ion pumps for the sustainability of life. It remains a significant challenge to develop high-performance artificial light-driven ion pumps for solar energy harvesting in all solid-state materials. Here, we exploit the benefits of short channel lengths and efficient light absorption to demonstrate efficient photo-driven ion transport in slightly reduced vertical graphene oxide membranes (GOMs). Remarkably, this photo-driven ion pump exhibits excellent ability, countering a 10-fold electrolyte concentration gradient. We propose a plausible mechanism where light illumination enhances the electric potential of ion channels on GOMs triggered by the separation of photoexcited charge carriers between the sp2 and sp3 carbon clusters. This results in the establishment of an electric potential difference across the effective ion channels composed of sp3 carbon clusters, thus driving the directional transport of cations from the illuminated side to the non-illuminated side. The promising results of this study provide new possibilities for the application of vertical 2D nanofluidic membranes in areas such as artificial photosynthesis, light harvesting, and water treatment.

Highly efficient visible-light induced N-doped ZnO@g-C3N4 and S-doped ZnO@g-C3N4 photocatalysts for environmental remediation.

Facile, cost-effective and eco-friendly synthesis of N-doped ZnO@g-C3N4 and S-doped ZnO@g-C3N4 photocatalysts towards efficient degradation of environmental pollutants was achieved. The as-synthesized 2 wt% N-doped ZnO@g-C3N4 and 2 wt% S-doped ZnO@g-C3N4 achieved 96.2% and 90.4% degradation efficiencies towards crystal violet (100 ppm) within 45 min irradiation and 99.3% and 92.3% photocatalytic degradation efficiencies towards brilliant green (100 ppm) dye within 30 min irradiation, respectively, under a normal 90 W LED light instead of an expensive commercial light source. Moreover, the N-doped ZnO@g-C3N4 and S-doped ZnO@g-C3N4 nanocomposites showed excellent stability in the photodegradation of crystal violet and brilliant green dyes. The modification made on ZnO by doping with nitrogen and sulphur enhances the visible-light absorption as well as the separation of photoexcited charge carriers. The active radicals ˙OH and ˙O2- are both identified to play important roles in the photodegradation of crystal violet and brilliant green.

Synergistic atom co-sharing and S-scheme heterojunction: constructing Cu/CuO/Cu2O with ultrathin graphene-like carbon derived from basil seeds for enhanced photo-oxidation of benzyl alcohols to aldehydes.

This study is centered on the oxidative transformation of alcohols into their respective aldehyde compounds, employing an S-scheme heterostructure featuring CuO/Cu2O on graphene-like carbon (GLC) derived from a basil seed hydrogel. Experimental characterization and theoretical calculations highlight that the implementation of S-scheme heterostructures achieves not only enhanced charge-separation efficiency, facilitated by the interfacial built-in electric field, Cu co-sharing at the CuO/Cu2O interface, and electron carrier activity of the GLC support, but also maintains a strong driving force for photocatalytic organic conversion. The resulting nanocomposites play a crucial role in transferring and reducing the recombination of photoexcited charge carriers, preserving the oxidizability of CuO holes and the reducibility of Cu2O electrons. Through meticulous adjustment of precursor amounts, the CuO-Cu2O/GLC heterojunction exhibited the highest photocurrent at 6.83 mA cm-2, demonstrating optimal performance in the photocatalytic selective oxidation of benzyl alcohol with an average conversion rate of 95.0%. Furthermore, the stability of CuO-Cu2O/GLC was thoroughly investigated, revealing sustained high conversion even after five repeated experiments, underscoring its potential for practical applications. The study also proposes a plausible mechanism for the transformation of benzyl alcohol into benzaldehyde through capture experiments of active species. Importantly, this research introduces a straightforward in situ hydrothermal growth protocol for efficiently constructing metal oxide heterostructures wrapped in an rGO support. It provides valuable insights into designing new synthetic strategies for preparing efficient photocatalysts and hints at the development of novel, efficient, and practical photocatalytic systems.

On the performance of vertical MoS2 nanoflakes as a photoelectrochemical studies for energy application

With the help of a hydrothermal process, we were able to prepare vertically layered MoS2 nanoflakes that were rooted to TiO2 modified. MoS2 nanoflakes and TiO2 contribute significantly to the strong XRD peaks and μ-Raman spectroscopy, and this phenomenon may also be explained by the unique structure of vertically stacked MoS2 nanoflakes on TiO2 that has many exposed edges and large surfaces as well as high electron transfer rates between TiO2 and MoS2. As can be clearly seen, there are no noticeable changes in the self-photodegradation of MB under visible light interaction (VLI), and the MoS2 doped TiO2 photocatalyst displays ∼ 90 % degradation efficiency. By, measuring photoelectrochemically, charge carriers are separated efficiently. These experiments illustrate the transient photocurrent response of the MoS2 doped TiO2 photocatalyst while cycling between three on/off cycles. As a result of a low recombination rate of the photoexcited charge carriers, the MoS2 doped TiO2 photocatalyst displays superior photocurrent response. In other words, a lower charge transfer resistance results in a faster transfer of charge between the surfaces.

Eco-benign synthesis of α-Fe2O3 mediated Trachyspermum ammi: A new insight to photocatalytic and bio-medical applications

Environmental contaminants are a golabl concern, and even at very low quantities, they can be extremely harmful to the aquatic ecosystem. Microbial resistance and photocatalysis are popular strategies for removing hazardous substances from the environment. The present study developed the green synthesis of α-Fe2O3 nanoparticles from Trachyspermum ammi plant leaf extract. The crystalline and structural stability of green synthesized α-Fe2O3 nanoparticles were studied by XRD. The surface morphology was investigated by TEM and FESEM equipped with EDX analysis. Metal, oxygen bond (Fe-O) and their interaction on the surfaces and their valency were explored from FTIR and XPS spectra. Optical orientations and electron movements were revealed from UV-DRS analysis. Visible light photocatalytic methylene blue degradation confirmed the largest surface area, rate of recombination, photo-excited charge carriers, photo-sensitivity range and radical generations of α-Fe2O3 nanoparticles. The green synthesized α-Fe2O3 nanoparticles bacterial susceptibility was examined by the well diffusion method using (10, 20, 50, 100) µg/L concentration of nanoparticles. The antibacterial activity enhanced with the higher concentration of α-Fe2O3 (100 µg/L). The gram-negative bacteria of E.coli has high bacterial susceptibility on α-Fe2O3 nanoparticles. The reactive oxygen species and OH radicals of α-Fe2O3 nanoparticles are demonstrated better resistivity against bacterial strains. The present study, therefore, suggested that the green synthesis would be an appropriate approach to synthesize the nanomaterials and confirming their suitability for use in industrial wastewater treatment and biomedical applications.

Cu2O/BiVO4 heterostructure controllably triggers radical and non-radical persulfate activation via light “on-off” for efficient organic contaminants degradation

The regulation of peroxydisulfate (PDS) activation pathway is crucial for its practical application in complicated water matrix. In this work, we have developed a Cu2O/BiVO4 (Cu-BVO) heterostructure to achieve controlled triggering of both radical and non-radical PDS activation by dexterously utilizing light “on-off” for highly efficient degradation of organic contaminants. In dark condition, Cu-BVO serves as an electron donor to facilitate the cleavage of O-O bond in PDS via a radical pathway to generate •OH and •O2-. Upon light irradiation, Cu-BVO unexpectedly acts as an electron acceptor in the reversed electron flow to generate a metastable surface complex (Cu-BVO-PDS*) via non-radical pathway. Analysis of the kinetic behavior of the excited electrons and holes suggests that the directional migration of photo-excited charge carriers in Cu-BVO results in the electron-deficient region on the Cu2O surface, which provides an ideal platform for the generation of Cu-BVO-PDS*.

PH-Dependent photocatalytic performance of faceted BiOBr semiconductor particles in degradation of dyes

Bismuth oxyhalides, such as bismuth-oxy-bromide (BiOBr), show high performance in photocatalytic oxidation of water pollutants. Process conditions, and in particular the pH of the solution, largely affect the photocatalytic efficacy, which is fundamentally poorly understood. We prepared {001} faceted bismuth-oxy-bromide (BiOBr), and determined by advanced AFM analysis that the surface charge of the {001} facet of BiOBr is slightly positive at acidic pH (pH 3) and significantly negative and increasing in negative surface charge in the order pH 6 < pH 9. Decomposition of MB or RhB by illumination of BiOBr is strongly favored by basic conditions (pH 9-10), as evident from discoloration experiments, and determination of the TOC as a function of time of illumination. Reference experiments show the pH dependent degradation profiles cannot be explained by differences in quantities of adsorption of the dye, but rather by the lifetime and/or quantity of BiOBr related photoexcited (surface) charge carriers (holes and electrons) – correlating with surface charge. Using MeOH and other scavengers, we show oxidation of MB or RhB is mainly induced by superoxide anions (•O2− radicals) at low pH and by holes or hydroxyl radicals at high pH – in agreement with surface charge induced band bending. This study provides novel understanding of the pH dependence of the rates of photocatalytic degradation of dyes using BiOBr photocatalysts.

Asymmetry of Structural and Electrophysical Properties of Symmetrical Titania Nanotubes as a Result of Modification with Barium Titanate

Anodic titania nanotubes (TiO2-NT) are very promising for use in photocatalysis and photovoltaics due to their developed surface, symmetrical structure and conductive properties, which, moreover, makes them a convenient matrix for creating various nanocomposites. Herein we propose a new facile way of synthesizing symmetrical TiO2-NT followed by a modification with barium titanate (BaTiO3) nanoparticles, combining the advantages of electrochemical oxidation and hydrothermal synthesis. The electrophysical and optoelectronic properties of the formed nanocomposites have been studied. An asymmetry of the current–voltage characteristics was revealed. It is shown that during the barium titanate deposition, a symmetry-breaking nanoheterojunction TiO2/BaTiO3 is formed. Using EPR spectroscopy, paramagnetic defects (titanium, barium and oxygen vacancies) in the samples were determined. It was observed for the first time that upon illumination of titania nanotubes modified with BaTiO3, the asymmetrical separation of photoexcited charge carriers (electrons and holes) between TiO2-NT and BaTiO3 occurs, followed by the capture of electrons and holes by defects. As a result, the photoinduced charge accumulates on the defects.

Comparison of Modification Methods of Photocatalyst G-C3N4 in Hydrogen Production Performance

Non-metallic polymer semiconductor graphite phase carbon nitride (g-C3N4) is a low-cost, environmentally friendly raw material with suitable bandgap width and stable chemical properties. However, there are still problems such as lower surface area, higher recombination rate of photo-excited charge carriers, and lower utilization rate of visible light. This paper introduces the structure, physicochemical properties, preparation methods, and modification ideas of graphite phase carbon nitride, focusing on several popular modification achievements in recent years, as well as a horizontal comparison of their respective production methods, mechanisms, and hydrogen production efficiency. Finally, the challenges and prospects in the modification of carbon nitride are discussed.

Integrating g-C3N4 nanosheets with MOF-derived porous CoFe2O4 to form an S-scheme heterojunction for efficient pollutant degradation via the synergy of photocatalysis and peroxymonosulfate activation

When confronted with wastewater that is characterized by complex composition, stable molecular structure, and high concentration, relying solely on photocatalytic technology proves inadequate in achieving satisfactory degradation results. Therefore, the integration of other highly efficient degradation techniques has emerged as a viable approach to address this challenge. Herein, a novel strategy was employed whereby the exfoliated g-C3N4 nanosheets (CNs) with exceptional photocatalytic performance, were intimately combined with porous rod-shaped cobalt ferrite (CFO) through a co-calcination process to form the composite CFO/CNs, which exhibited remarkable efficacy in the degradation of various organic pollutants through the combination of photocatalysis and Fenton-like process synergistically, exemplified by the representative case of tetracycline hydrochloride (TCH, 200 mL, 50 mg/L). Specifically, under 1 mM of peroxymonosulfate (PMS) and illumination conditions, 50 mg of 1CFO/9CNs achieved a TCH removal ratio of ∼90% after 60 min of treatment. Furthermore, this work comprehensively investigated the influence of various factors, including catalyst and PMS dosages, solution pH, and the presence of anions and humate, on the degradation efficiency of pollutants. Besides, quenching experiments and EPR tests confirmed the establishment of an S-scheme heterojunction between CNs and CFO, which facilitated the effective spatial separation of photoexcited charge carriers and preserved the potent redox potential of photogenerated electrons and holes. This work offers a valuable reference for the integration of photocatalysis with the PMS-based Fenton-like process.

Enhanced photoresponse of self-powered ZnO-based photoelectrochemical-type UV photodetectors via Ga-doping for optical communication application

Wide-bandgap metal-oxide semiconductors hold promise for self-powered photoelectrochemical (PEC) type UV photodetectors (PDs) due to their inherent photoelectric properties and durability. However, inadequate charge carrier separation and transport remain a fundamental challenge for practical applications. Herein, the extraordinary efficacy of introducing gallium (Ga) into ZnO nanowire to design high-performance PEC-type PDs is demonstrated. The obtained Ga-doped ZnO (ZnO:Ga) PEC-type photodetectors exhibit superior UV photoresponse performance, featuring an exceptionally high responsivity of 233.26mA/W, an excellent detectivity of 4.18 × 1012 Jones, and a fast response/recovery time of 159/150 ms under 365nm illumination without external bias voltages. These accomplishments outperform the majority of previously reported PEC-type UV PDs. It is found that the unique properties of ZnO:Ga nanowires, including improved donor density and steeper band bending at solid-liquid interface, significantly enhance the generation, transfer, and separation of photoexcited charge carriers, thus contributing to the superior photodetector’s performance. In addition, a secure UV communication system using international Morse code is set to demonstrate the feasibility of ZnO:Ga nanowires PEC-PD as a self-powered optical receiver. This study introduces a promising approach to develop self-powered photodetectors that are highly stable, cost-effective, and poised for integration into future multifunctional optoelectronic devices.

Breaking symmetry of carbon nitride skeleton by embedding pyridine ring to promote photocatalytic performance

Breaking symmetry of polymeric carbon nitride (CN) skeleton by heteroatomic substitution can induce in-plane polar electric field, thus promoting the migration and separation of photoexcited charge carriers. Herein, a carbon-doped CN (pyCN) by embedding pyridine ring was prepared through a skeleton modification strategy. Owing to the n → π* electron transition and the formed in-plane polar electric field, the obtained pyCN displayed obvious extended optical absorption and photoinduced charge migration efficiency in comparison with pure CN. The optimized photocatalytic H2 evolution rate over pyCN reached 242.9 μmol·h−1, which is 6 times of that over pure CN with TEOA as a sacrifice. Additionally, the wavelength for H2 evolution over pyCN extends to 550 nm while that is 450 nm over pure CN, demonstrating an improved efficiency of sunlight utilization after skeleton modification. In addition, pyCN showed synergistic oxidation of bisphenol A (BPA) during hydrogen generation under visible light. The conversion efficiency of BPA reached 18.7 % in 24 h according to the external standard analysis. This work provides an efficient approach for enhancing optical absorption and inhibiting photoexcited charge recombination for higher photocatalytic performance and wilder applications.

Photocatalytic Oxygen Evolution under Visible Light Mediated by Molecular Heterostructures.

Due to their structural and property tunability, semiconductive conjugated polymers (CPs) have emerged as promising candidates for photocatalytic water splitting. Compared with inorganic materials, the photocatalytic performance of mono-component polymers was limited by the fast recombination of photoexcited charge carriers, and they always needed to catch up to expectations. To this end, researchers established molecular donor-acceptor heterostructures, which could notably promote oxygen production efficiency due to their more effective charge carrier separation. In this work, easy Schiff base reactions between side-chain -CHO groups and terminal -NH2 groups were used to introduce benzene and perylene diimide (PDI) into the molecular heterostructure to serve as electron donors (D) and electron acceptors (A). In particular, for the first time, we employed the molecular heterostructures of CPs to promote photocatalytic O2 production. One prepared molecular heterostructure was demonstrated to improve oxygen generation rate (up to 0.53 mmol g-1 h-1) through visible light-driven water splitting. Interestingly, based on the photoelectric properties, a stepwise two-electron/two-electron pathway constituted the photocatalytic mechanism for oxygen production with the molecular heterostructure. These results provide insights into designing and fabricating high-performance molecular heterostructures for photocatalytic oxygen production.

Boosting solar driven hydrogen production rate of Cu2S@CdS p-n heterostructures and CuxCd1-xS nanorods

To improve CdS photocatalytic activity, both CuxCd1-xS nanorods and Cu2S@CdS p-n heterostructures have been prepared with a simple solvothermal method. The results showed that the maximum H2 generation rate is equal to 10 mmol h−1 g−1 and 12.1 mmol h−1 g−1 for samples 0.3-Cu2S@CdS and Cu0.1Cd0.9S, respectively, which are nearly 9 and 12 times larger than that of CdS (1.2 mmol h−1 g−1) without using Pt as the co-catalyst. The measurements of the light absorption revealed that the samples Cu2S@CdS p-n heterostructures and CuxCd1-xS nanorods have an increased visible light absorption. Band structure revealed that Cu2S@CdS p-n composite is a type-II heterostructure, which facilitates the separation of photoexcited charge carriers. Furthermore, as the Cu-doping CdS introduced a defect level, which induced the increase of carrier density, and the defect energy level in Cu0.1Cd0.9S and the band energy in CdS form a type-II heterostructure. Due to the appropriate band structure and stronger light absorption, Cu2S@CdS p-n heterostructures and Cu+-doping CdS demonstrated an improved photocatalytic activity and stability.

Physics of photovoltaic signal modifications in p–n photodiodes

The photovoltaic signal is an important characteristic of photodetectors, including but not limited to those that are based on p–n or p–i–n photodiodes. In an open-circuit configuration, pulsed excitation of the detector with ultrafast (femto or nanosecond) pulses leads to a photovoltaic signal that decays slowly (micro-second time scale). If the physics in the detector is dominated by the recombination of the photo-excited charge carriers, one expects the signal to decay without changing its sign. However, some experiments using short-pulse excitation have found that photovoltaic signals can undergo a sign change as a function of time following excitation, with positive signals immediately following the excitation, turning to negative signals several microseconds later. Here, we study various physical effects (density, temperature, electrostriction, pressure, photostriction, and bandgap renormalization) and determine their effect on photovoltaic signals. If, following ultrafast excitation, the carrier density and temperature are increased, and during relaxation the system reaches a state sufficiently close to the quasi-thermal equilibrium in which the carrier density is still elevated, but smaller than the intrinsic thermal equilibrium density at the elevated temperature, then the signal can become negative.