Photoexcited Holes Research Articles

CQDs modulating Z-scheme g-C3N4/BiOBr heterostructure for photocatalytic removing RhB, BPA and TC and E. coli by LED light

To improve the photocatalytic performance, it is necessary to reduce the combination of photo-induced charges and improve the absorbance intensity of visible light. Herein, we have reported novel heterojunctions of indirect Z-scheme CQDs/g-C3N4/BiOBr composites (noted as CN@BOB-CQDs). Various techniques proved the successful preparation of heterojunction structure between CQDs, g-C3N4 and BiOBr. Compared with g-C3N4 and BiOBr, the composites presented excellent performance in removing Rhodamine B (RhB), bisphenol A (BPA) and tetracycline (TC) and E. coli by LED light illumination. The improvement of photocatalytic properties was assigned to its good separation efficiency of generated charges and strong redox ability of its photo-excited electrons (e) and holes (h+), which realized from the Z-type photo-excited charges transition in CN@BOB-CQDs composite. Besides, CQDs could promote the transmission of electrons and enhance the absorbance intensity of visible light. Finally based on the radicals’ trapping experiments and the semiconductor energy band position, the photocatalytic mechanism on the degradation of RhB was illustrated.

Ultrafast and Highly Sensitive Dual-Channel FET Photodetector Based on a Two-Dimensional MoS2 Homojunction.

Two-dimensional transition metal dichalcogenide-based phototransistors have been intensively studied in recent years due to their high detection rate and flexibility. However, the photogating effect, usually appearing in the devices, leads to a poor transient photoresponse, which slows down the imaging rate of the camera based on the devices. Here, we demonstrate a dual-channel two-dimensional field-effect phototransistor composed of a vertical molybdenum disulfide (MoS2) p-n homojunction as the sensitizing channel layer. Owing to the effective separation by the vertical built-in electric field and rapid migration of photoexcited electrons and holes in the separated channels, the fabricated dual-channel FET device simultaneously exhibits prominent responsivity and greatly improved time response in comparison to the pristine MoS2 FET detectors. Excellent device performance has been achieved, with a responsivity of 3.4 × 104 A/W at a source-drain voltage (VDS) of 1 V, corresponding to a detectivity (D*) of 1.9 × 1013 Jones@532 nm and a gain of more than 105 electrons per photon, an external quantum efficiency of 9.6%, and a response time of tens of milliseconds. Especially, the response time of the dual-channel FET device is 3 orders of magnitude faster than that of the pristine device. Our results provide a new way to overcome the inherent photogating drawback of two-dimensional FET optoelectronic devices and to develop a related high frame rate imaging system.

Defect coupled MoSx sites over ZnIn2S4 nanosheets towards efficient H2 evolution

The inefficient separation of photoexcited electrons and holes as well as the lack of sufficient reactive sites are considered as the major factors hindering the catalytic activity of the semiconductors. Herein, we report an environmentally friendly and energy-saving method to realize the co-deposition of amorphous MoSx onto ZnIn2S4 (ZIS) nanosheets, through which effectively enhanced performance of H2 generation from water reduction can be achieved. Furtherly, defect-poor ZIS and defect-rich O-doped ZIS (O-ZIS) were taken as the fabrication matrixes respectively, and larger enhancement resulting from MoSx loading over O-ZIS than that over ZIS was achieved, suggesting the synergistic effect that one plus one is greater than two, primarily due to the remarkably enhanced charge carriers separation and more abundant reaction sites for H2 production resulting from the defect engineering coupled surface active sites constructing. Notably, the optimal MoSx/O-ZIS achieved the H2 evolution rate of 15.32 mmol g−1 h−1, which is 17.8 and 8.5 times as that achieved over blank ZIS and O-ZIS, respectively. This work furnishes a facile paradigm that efficient composite materials can be facilely controlled towards boosted solar energy conversion, which significantly simplifies the activity majorization of the component-designed MoSx/ZIS photocatalysts.

Magnetic and Optical Field Multi-Assisted Li-O2 Batteries with Ultrahigh Energy Efficiency and Cycle Stability.

The photoassisted lithium-oxygen (Li-O2 ) system has emerged as an important direction for future development by effectively reducing the large overpotential in Li-O2 batteries. However, the advancement is greatly hindered by the rapidly recombined photoexcited electrons and holes upon the discharging and charging processes. Herein, a breakthrough is made in overcoming these challenges by developing a new magnetic and optical field multi-assisted Li-O2 battery with 3D porous NiO nanosheets on the Ni foam (NiO/FNi) as a photoelectrode. Under illumination, the photogenerated electrons and holes of the NiO/FNi photoelectrode play a key role in reducing the overpotential during discharging and charging, respectively. By introducing the external magnetic field, the Lorentz force acts oppositely on the photogenerated electrons and holes, thereby suppressing the recombination of charge carriers. The magnetic and optical field multi-assisted Li-O2 battery achieves an ultralow charge potential of 2.73 V, a high energy efficiency of 96.7%, and good cycling stability. This external magnetic and optical field multi-assisted technology paves a new way of developing high-performance Li-O2 batteries and other energy storage systems.

Decoration of BiVO4 Photoanodes with Near-Infrared Quantum Dots for Boosted Photoelectrochemical Water Oxidation.

Broadening light absorption and improving charge carrier separation are very critical to boost the water splitting efficiency in photoelectrochemical (PEC) systems. We herein reported a heterostructured photoanode consisting of BiVO4 and eco-friendly, near-infrared (NIR) CuInSeS@ZnS core-shell quantum dots (QDs) for PEC water oxidation. The decoration of core-shell QDs concurrently extends the absorption range of BiVO4 from the ultraviolet-visible to NIR region and promotes the effective separation and transfer of photo-excited electrons and holes. Without any sacrificial agents and co-catalysts, the as-fabricated NIR core-shell QDs/BiVO4 heterostructured photoanodes exhibit an approximately fourfold higher photocurrent density than that of the bare BiVO4, up to 3.17 mA cm-2 at 1.23 V versus the reversible hydrogen electrode. It is revealed that both a suitable band alignment and an intimate interfacial junction between QDs and BiVO4 are the main factors that result in enhanced charge separation and transfer efficiencies. We also highlight that the NIR CISeS QDs passivated with a ZnS shell can suppress the non-radiative recombination and enhance the stability of the QD photoanodes for optimized PEC performance. This work provides a facile and effective approach to boost the water oxidation efficiency of semiconductor photoanodes via utilizing NIR core-shell QDs as a light sensitizer and charge carrier separator.

CuWO4/CuS heterojunction photocatalyst for the application of visible-light-driven photodegradation of dye pollutions

Water pollution is a very serious problem concerning human health, and organic dyes are important pollution sources. Photocatalysts could convert light energy into chemical energy and photodegrade organic dyes. Thus, their potential use has been considered important. Herein, a copper-based heterojunction photocatalyst was developed through the hydrothermal method. In the heterojunction, two copper-based nanostructures, namely, nanodots CuWO4 and nanorods CuS, were used to form a composite, and all products with different ratios showed better performance for the photodegradation of Rhodamine B (RhB), especially at the ratio of 1:1 (CuWS), which possessed the best photocatalytic efficacy. Narrow bandgap, large specific surface area and efficiency of the separation of photoexcited electrons and holes were the essential factors for the enhanced photocatalytic activity. Based on the analysis of the active species trapping experiments and electron spin resonance spectra, the photogenerated holes and •O2¯ were the essential active species during the photocatalytic degradation of RhB. Additionally, the sample CuWS could effectively photocatalytic degraded many other types of organic dyes under visible light and shows the great potential of application on environmental governance.

Ultrathin ZnIn2S4 nanosheets decorating PPy nanotubes toward simultaneous photocatalytic H2 production and 1,4-benzenedimethanol valorization

It is highly desirable to make full use of photogenerated charge carriers by elaborately designing high-efficiency multifunctional photocatalysts with achieving bi-value-added production. Herein, we report the photocatalyst with ultrathin ZnIn2S4 nanosheets decorating polypyrrole nanotubes, thanks to the matched bandgap, is capable of harvesting visible light for photocatalysis, during which the photoexcited electrons and holes are used for H2 evolution and 1,4-benzenedimethanol (BDM) upgrading oxidation, respectively. Comprehensive experiments and density functional theory calculations indicate that the photoinduced electrons tend to be transferred to the inner PPy nanotubes of PPy@ZIS composite for photocatalytic H2 generation, while the photoexcited holes favorably oxidize BDM into 1,4-phthalaldehyde (PAD) on outer ZIS nanosheets of PPy@ZIS composite. The optimized PPy@ZIS exhibits an apparent quantum efficiency of 6.43% (at 420 nm) for photocatalytic H2 evolution and BDM valorization into PAD at a rate of 735 μmol g−1 h−1, remarkably promoting the utilization efficiency of charge carriers.

Novel Fe2O3/{101}TiO2 nanosheet array films with stable hydrophobicity and enhanced photoelectrochemical performance

Single crystal TiO2 nanosheet (TNS) arrays with coexposed {101} and {001} facets were synthesized on FTO conductive glass directly, thorn-like Fe2O3 were deposited on the {101} surface of TNS by a chemical bath deposition method. Taking advantage of different energy band positions of {101} and {001} facets, the higher band offset value between Fe2O3 and {101} facets was achieved, which would accelerate the photoexcited electrons and holes separation between Fe2O3 and {101} facets of TNS and enhance photoelectrochemical properties. Furthermore, without modification by low energy chemicals, such novel Fe2O3/TNS heterostructure electrodes exhibited stability hydrophobicity. After UV irradiation for 240 min, the contact angle of the Fe2O3/TNS (80min) films retained hydrophobicity (134.8°), which is approximately 7.1 times higher than that of bare TNS films. The optimal composite photoelectrode exhibited the highest transient photocurrent intensity (48 μA), which is about 4.4 times superior over the bare TNS. The superior photoelectrochemical properties and stability hydrophobicity were attributed to the suitable band edge positions and hydrophobic thorn-like α-Fe2O3.

Synergistic poly(lactic acid) photoreforming and H2 generation over ternary NixCo1-xP/reduced graphene oxide/g-C3N4 composite

Effective utilization of photoexcited electrons and holes is always a challenge in photocatalytic reactions. Herein, we reported ternary NixCo1-xP/reduced graphene oxide/g-C3N4 (NixCo1-xP/rGO/CN) composite as a photocatalyst for synergistic poly(lactic acid) photoreforming and H2 generation in alkaline aqueous solution. The rate of H2 production over the optimal 15Ni0·1Co0·9P/rGO/CN reached 576.7 μmol h−1 g−1, which is 3.6 times as high as binary 15Ni0·1Co0·9P/CN composite. The apparent quantum efficiency of the optimal 15Ni0·1Co0·9P/rGO/CN was 1.7% at λ = 420 nm monochromatic light. Mott–Schottky analysis suggested that the photogenerated electrons transfer along the pathway of CN→rGO→Ni0·1Co0·9P, where rGO and Ni0·1Co0·9P functioned as the medium for electron transporting and reaction site for H2 generation, respectively. Meanwhile, poly(lactic acid) was photoreformed into formate and acetate by the photogenerated holes and hydroxyl radical. This work demonstrates that ternary NixCo1-xP/rGO/CN composite can be applied as a cheap and promising photocatalyst for synergistic plastic photoreforming and H2 generation.

Cooperative Coupling of Oxidative Organic Synthesis and Hydrogen Production over Semiconductor-Based Photocatalysts.

Merging hydrogen (H2) evolution with oxidative organic synthesis in a semiconductor-mediated photoredox reaction is extremely attractive because the clean H2 fuel and high-value chemicals can be coproduced under mild conditions using light as the sole energy input. Following this dual-functional photocatalytic strategy, a dreamlike reaction pathway for constructing C-C/C-X (X = C, N, O, S) bonds from abundant and readily available X-H bond-containing compounds with concomitant release of H2 can be readily fulfilled without the need of external chemical reagents, thus offering a green and fascinating organic synthetic strategy. In this review, we begin by presenting a concise overview on the general background of traditional photocatalytic H2 production and then focus on the fundamental principles of cooperative photoredox coupling of selective organic synthesis and H2 production by simultaneous utilization of photoexcited electrons and holes over semiconductor-based catalysts to meet the economic and sustainability goal. Thereafter, we put dedicated emphasis on recent key progress of cooperative photoredox coupling of H2 production and various selective organic transformations, including selective alcohol oxidation, selective methane conversion, amines oxidative coupling, oxidative cross-coupling, cyclic alkanes dehydrogenation, reforming of lignocellulosic biomass, and so on. Finally, the remaining challenges and future perspectives in this flourishing area have been critically discussed. It is anticipated that this review will provide enlightening guidance on the rational design of such dual-functional photoredox reaction system, thereby stimulating the development of economical and environmentally benign solar fuel generation and organic synthesis of value-added fine chemicals.

Piezoelectric polarization promoted spatial separation of photoexcited electrons and holes in two-dimensional g-C3N4 nanosheets for efficient elimination of chlorophenols

Graphitic carbon nitride (g-C3N4) has been proved to be a potential photocatalyst for environment purification, but the high recombination rate of photogenerated carriers leads to the low photocatalytic efficiency. Herein, we report the enhanced degradation of chlorophenols by 2D ultrathin g-C3N4 nanosheets with intrinsic piezoelectricity through photopiezocatalysis strategy. Under the simultaneous visible-light irradiation and ultrasonic vibration, the 2D g-C3N4 presented improved removal efficiency for elimination of 2,4-dichlorophenol (2,4-DCP) with an apparent rate constant of 6.65 × 10−2 min−1, which was 6.7 and 2.2 times of the photocatalysis and piezocatalysis, respectively. The improved removal efficiency was attributed to the sufficient separation of free charges driven by the ultrasound-induced piezoelectric field in the 2D g-C3N4, which was demonstrated by the enhanced current response under photopiezocatalysis mode. Additionally, the photopiezocatalysis of 2D g-C3N4 was proved to possess well universality for removing different chlorophenols, as well as high durability and dechlorination efficiency. Finally, a possible photopiezocatalytic mechanism for removal of 2,4-DCP was proposed based on the electron paramagnetic resonance (EPR) technique and the determination of intermediates through liquid chromatography-mass spectrometry (LC-MS) analysis. This work provides a promising strategy for the design of energy-conversion materials towards capturing solar and mechanical energy in ambient environment.

The critical role of hot carrier cooling in optically excited structural transitions

The hot carrier cooling occurs in most photoexcitation-induced phase transitions (PIPTs), but its role has often been neglected in many theoretical simulations as well as in proposed mechanisms. Here, by including the previously ignored hot carrier cooling in real-time time-dependent density functional theory (rt-TDDFT) simulations, we investigated the role of hot carrier cooling in PIPTs. Taking IrTe2 as an example, we reveal that the cooling of hot electrons from the higher energy levels of spatially extended states to the lower energy levels of the localized Ir–Ir dimer antibonding states strengthens remarkably the atomic driving forces and enhances atomic kinetic energy. These two factors combine to dissolute the Ir–Ir dimers on a timescale near the limit of atomic motions, thus initiating a deterministic kinetic phase transition. We further demonstrate that the subsequent cooling induces nonradiative recombination of photoexcited electrons and holes, leading to the ultrafast recovery of the Ir–Ir dimers observed experimentally. These findings provide a complete picture of the atomic dynamics in optically excited structural phase transitions.

Light-assisted fermentative hydrogen production in an intimately-coupled inorganic-bio hybrid with self-assembled nanoparticles

Photosensitizing bacterial cells with semiconductor nanoparticles is an attractive way to enhance fermentative hydrogen production metabolism via harnessing solar energy as extra driving force, but the enhancement degree is typically limited by inefficient utilization of extracellular photo-electrons. Here, an intimately-coupled inorganic-bio hybrid with enhanced hydrogen-producing activity was constructed by utilizing self-assembled selenide/ sulfide semiconductor nanoparticles (CdSexS1-x) in Escherichia coli as the light harvester. Due to circumvention of transmembrane electron transfer limitation, this hybrid exhibited 2.6-fold higher hydrogen production rate than those with extracellular nanoparticles and achieved light energy conversion efficiency as high as 27.6%. The incorporation of photocatalysis did not impair the bacterial viability, attributed to an efficient scavenging of photo-excited holes by metabolites (e.g., lactate) and minimal reactive oxygen species production. Overall, the light-assisted fermentation system developed in this study offer opportunities for sustainable production of bio-hydrogen and may be extended to bio-photocatalytic production of other valuable chemicals.

Light-Tunable Surface State and Hybridization Gap in Magnetic Topological Insulator MnBi8Te13.

MnBi8Te13 is an intrinsic ferromagnetic (FM) topological insulator with different complex surface terminations. Resolving the electronic structures of different termination surfaces and manipulation of the electronic state are important. Here, by using micrometer spot time- and angle-resolved photoemission spectroscopy (μ-TrARPES), we resolve the electronic structures and reveal the ultrafast dynamics upon photoexcitation. Photoinduced filling of the surface state hybridization gap is observed for the Bi2Te3 quintuple layer directly above MnBi2Te4 accompanied by a nontrivial shift of the surface state, suggesting light-tunable interlayer interaction. Relaxation of photoexcited electrons and holes is observed within 1-2 ps. Our work reveals photoexcitation as a potential control knob for tailoring the interlayer interaction and surface state of MnBi8Te13.

Evaluation of practical application potential of a photocatalyst: Ultimate apparent photocatalytic activity

Suffered from rapid recombination of electrons and holes, apparent photocatalytic activity (APA) of all photocatalysts can never achieve their theoretical ultimate values. But the upper limit of practical APA is of great significance to evaluate the practical application potential of a photocatalyst. Thus, in this work, the concept of ultimate apparent photocatalytic activity (UAPA) was firstly proposed and a convenient evaluation method was first established based on the nature that EDTA-2Na can exclusively scavenge photo-excited holes, and methyl orange (MO) is mainly attacked by superoxide radical (O2−) which is produced instantly by photo-excited electrons. From a macro perspective, six popular photocatalysts were designedly selected to verify the feasibility and application scope of the proposed UAPA evaluation method. Moreover, O2− production rate and photocurrent intensity were measured by spectroscopy and spectrum analyses, and theoretical carrier concentrations were calculated by density functional theory (DFT) to further confirm the rationality and reliability of the proposed method. Positive responses of all the tests guarantee that the proposed UAPA could precisely evaluate the application potential of a photocatalyst and rank the photocatalysts according to their practical potential.

Modulating Local Charge Distribution of Carbon Nitride for Promoting Exciton Dissociation and Charge-Induced Reactions.

The sustainable light can generate reduction and oxidation centers in situ through the generation of photoexcited electrons and holes in the presence of photocatalyst. However, the photoexcited electrons and holes have huge Coulombic attraction and high exciton binding energy due to the weak screening effect and dielectric properties in many low-dimensional conjugated polymers, such as carbon nitride. Reducing the exciton binding energy of carbon nitride and promoting the conversion of excitons into free charge carriers are necessary for improving the activity of photocatalytic reactions but still very challenging. Here, by introducing amino-cyano functional groups into carbon nitride, it is demonstrated that excitons can be effectively dissociated into electrons and holes by finely controlling the charge distribution of heptazine ring. It is found that carbon nitride with heptazine rings of positive charge distribution can greatly reduce the exciton binding energy to 24 from 71 meV. Compared with heptazine ring having negative charge distribution, heptazine ring with positive charge distribution can increase photocatalytic hydrogen production of carbon nitride by up to ten times. This work provides an easy way to promote the dissociation of excitons in carbon nitride by regulating the charge distribution.

Minimized Pt deposition on CdS simultaneously maximizes the performance of hydrogen production and aromatic alcohols oxidation

Constructing coupling reaction system is an effective strategy to simultaneously improve the utilization of photoexcited electrons and holes. Herein, a series of Pt/CdS photocatalysts with minimized Pt were developed by a novel method and used for the coupling system of hydrogen production and aromatic alcohols oxidation. The optimized 0.045%-Pt/CdS exhibits enhanced production rates of H2 and aromatic aldehydes of 4.87 and 5.54 mmol·gcat−1·h−1 respectively, which is 325 and 277 times higher than those of pure CdS. Furthermore, the apparent quantum efficiency (AQE) of hydrogen production is up to 28.9% at 450 nm without palpable decrease in photoactivity during 18 h continuous testing. The enhanced photocatalytic activity is mainly attributed to the synergistic roles of Pt and CdS: (1) the larger interfacial area formed by the tight anchorage of Pt on CdS is more favorable for transferring photoexcited electrons (e−) to Pt, while metallic Pt ensures outstanding e− mobility and offers more active sites for reducing H+ to H2; (2) the photoexcited holes (h+) left on CdS oxidize aromatic alcohols into corresponding aldehydes, thereby achieving the simultaneous utilization of photoexcited h+ and e− in one reaction system. The anchoring design can also be expanded to other coupling systems.

Novel Two-Dimensional Janus MoSiGeN4 and WSiGeN4 as Highly Efficient Photocatalysts for Spontaneous Overall Water Splitting.

Searching for highly efficient and eco-friendly photocatalysts for water splitting is essential for renewable conversion and storage of inexhaustible solar energy but remains a great challenge. Herein, based on the new emerging two-dimensional (2D) material of MoSi2N4, we report novel Janus MoSiGeN4 and WSiGeN4 structures with excellent stabilities and great potentials in photocatalytic applications through first-principles calculations. Comprehensive studies show that MoSi2N4, MoSiGeN4, and WSiGeN4 exhibit semiconductor characteristics with an indirect gap, appropriate band gaps, and strong optical absorbance in the visible spectrum. Excitingly, by constructing Janus structures, an intrinsic electric field is realized that enhances the spatial separation and anisotropic migration of photoexcited electrons and holes. Further, this strategy can also alter the band alignment to provide an adequate photoexcited carrier driving force for water redox reactions. Moreover, the surface N vacancy can effectively lower the energy demand of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) so that the catalytic process can be self-sustained under the potential provided by the photocatalyst alone. Particularly, the overall water splitting can proceed simultaneously and spontaneously on the surface of MoSiGeN4 and WSiGeN4 when pH is 3 or ≥8, respectively. These explorations offer new prospects for the design of highly efficient photocatalysts.

Terahertz Photoconductivity Spectra of Electrodeposited Thin Bi Films.

Electron dynamics in the polycrystalline bismuth films were investigated by measuring emitted terahertz (THz) radiation pulses after their photoexcitation by tunable wavelength femtosecond duration optical pulses. Bi films were grown on metallic Au, Pt, and Ag substrates by the electrodeposition method with the Triton X-100 electrolyte additive, which allowed us to obtain more uniform films with consistent grain sizes on any substrate. It was shown that THz pulses are generated due to the spatial separation of photoexcited electrons and holes diffusing from the illuminated surface at different rates. The THz photoconductivity spectra analysis has led to a conclusion that the thermalization of more mobile carriers (electrons) is dominated by the carrier–carrier scattering rather than by their interaction with the lattice.

Unraveling the Origin of Photocatalytic Deactivation in CeO2/Nb2O5 Heterostructure Systems during Methanol Oxidation: Insight into the Role of Cerium Species.

The study provides deep insight into the origin of photocatalytic deactivation of Nb2O5 after modification with ceria. Of particular interest was to fully understand the role of ceria species in diminishing the photocatalytic performance of CeO2/Nb2O5 heterostructures. For this purpose, ceria was loaded on niobia surfaces by wet impregnation. The as-prepared materials were characterized by powder X-ray diffraction, nitrogen physisorption, UV–visible spectroscopy, X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and photoluminescence measurements. Photocatalytic activity of parent metal oxides (i.e., Nb2O5 and CeO2) and as-prepared CeO2/Nb2O5 heterostructures with different ceria loadings were tested in methanol photooxidation, a model gas-phase reaction. Deep insight into the photocatalytic process provided by operando-IR techniques combined with results of photoluminescence studies revealed that deactivation of CeO2/Nb2O5 heterostructures resulted from increased recombination of photo-excited electrons and holes. The main factor contributing to more efficient recombination of the charge carriers in the heterostructures was the ultrafine size of the ceria species. The presence of such highly dispersed ceria species on the niobia surface provided a strong interface between these two semiconductors, enabling efficient charge transfer from Nb2O5 to CeO2. However, the ceria species supported on niobia exhibited a high defect site concentration, which acted as highly active recombination centers for the photo-induced charge carriers.