Efficient Separation Of Photoexcited Carriers Research Articles

Ultrasensitive photodetector based on 2D WS2/AgInGaS quantum dots heterojunction with interfacial charge transfer

Tungsten disulfide (WS2), as a typical member of transition metal chalcogenides (TMDs), has attracted extensive research interest in optoelectronics, especially photodetectors. However, the performance of photodetectors based on monolayer WS2 is restricted to weak light absorption. Here, AgInGaS quantum dots (AIGS-QDs) with a large absorption coefficient and high quantum efficiency are integrated onto WS2 atomic layers to achieve excellent photoelectric performance. Notably, the observed photoluminescence (PL) quenching and the reduction of the decay time of PL in the WS2/AIGS-QDs heterojunction confirm the interfacial charge transfer from AIGS-QDs to WS2 layer. The results show that type II energy band arrangement leads to the efficient separation of photoexcited carriers at the interface between WS2 and AIGS-QDs. This WS2/AIGS-QDs photodetector achieves an ultrahigh responsivity (R) of 3.3 × 103 A W−1, an external quantum efficiency (EQE) of 7.8 × 106% and a detectivity (D*) of 1.3 × 1013 Jones. Our work provides promising potential for future high-performance monolayer TMD-based photodetectors.

Size of electron polarons in anatase TiO2 and their role in photocatalysis

Experiments detect small electron polarons in rutile-${\mathrm{TiO}}_{2}$, but only large ones in anatase. By using an accurate hybrid functional, we show that the ``large polaron'' in anatase is actually of medium size and can be obtained in a static calculation. These states are two-dimensional in the (001) plane and can get trapped at the (101) surface (where small polarons may exist too). The calculated binding energy and the simulated scanning tunneling microscopic image are in good agreement with the experimental observation. The fact that holes are trapped in small polaron states in the bulk while electrons are only at the surface means an efficient separation of photoexcited carriers, and is a factor in the excellent photocatalytic behavior of anatase.

CuS anchored onto peanut-like BiVO4 p-n heterojunction with enhanced photocatalytic degradation activity under visible light irradiation

• Peanut-like CuS/BiVO 4 p-n heterojunctions were prepared by two-step hydrothermal method. • CuS/BiVO 4 composites exhibit high activity of glyphosate degradation under visible light. • The reaction rate constant of 8%CuS/BiVO 4 is 3.8 times greater than that of BiVO 4 . • CuS/BiVO 4 strongly promotes the separation of photoexcited electron and hole pairs. Seeking cheap photocatalysts for practical application is a great challenge. It is greatly significant to construct heterojunctions for the efficient separation of photoexcited carriers. Herein, a peanut-like CuS/BiVO 4 p-n heterojunction is prepared using thiourea as the raw material via a two-step hydrothermal process. The 8%CuS/BiVO 4 composite has many advantages, including excellent visible light absorption capacity, photoelectric performance, and enhanced glyphosate degradation rate by 3.8 times compared to BiVO 4 . The probable photocatalytic mechanism is proposed according to the XPS, DRS, photocurrent, PL, and experimental results. The intimate interface contact between CuS and BiVO 4 effectively achieves quick separation of photoinduced charge carriers and improves photocatalytic activity, thus enhancing degradation efficiency of glyphosate. The glyphosate degradation path is also investigated according to the intermediates detected by mass spectrometry.

Flower-globular BiOI/BiVO4/g-C3N4 with a dual Z-scheme heterojunction for highly efficient degradation of antibiotics under visible light

A novel BiOI/BiVO4/g-C3N4 composite photocatalyst was successfully prepared via a simple ultrasonic-assisted hydrothermal method. The crystal structure, element composition, micro-morphology and optical performance of BiOI/BiVO4/g-C3N4 were analyzed. All composites showed better photocatalytic activity than pure BiVO4 in degrading levofloxacin hydrochloride (LEF). The kinetic constant of BiOI/BiVO4/g-C3N4 (0.0152 min−1) was 6.91, 3.10, 2.20 and 1.60 times that of g-C3N4 (0.0022 min−1), BiVO4 (0.0049 min−1), BiOI (0.0069 min−1) and BiVO4/g-C3N4 (0.0095 min−1), respectively. The degradation rate of LEF can reach 89.01% under optimal conditions. In addition, the influences of different pH values, inorganic salt ions and different kinds of antibiotics, binary and ternary compound antibiotics on photocatalytic activity were studied under visible-light driving. The photocatalyst showed excellent stability after being recycled three times. Three possible degradation pathways of LEF were analyzed by liquid chromatography-mass spectrometry (LC-MS). A possible mechanism of dual Z-scheme heterojunction electron transfer was put forward in this work. This transformation mode of electrons promotes the efficient separation of photo-excited carriers, which is also the reason why the photocatalytic activity is improved after constructing the heterostructure. The prepared composite exhibited eminent application prospects in the field of antibiotic wastewater treatment.

Facile construction of novel Bi2WO6/Ta3N5 Z-scheme heterojunction nanofibers for efficient degradation of harmful pharmaceutical pollutants

Pharmaceutical wastewater has become a severe, tremendous threaten to ecological environment and human health. Semiconductor photocatalysts have emerged as potential candidates for degrading pharmaceutical pollutants. Construction of highly efficient, stable and recyclable Z-scheme photocatalysts that are superior to individual constituents or widely studied heterojunction photocatalysts is very fascinating yet challenging. Herein, we report an efficient, stable and recyclable visible-light-driven (VLD) Bi2WO6/Ta3N5 Z-scheme heterojunction with compact interface contact fabricated via an electrospinning–calcination–solvothermal route, in which abundant Bi2WO6 nanosheets are in-situ, compactly and vertically grown on the surface of the Ta3N5 nanofibers. These as-fabricated Z-scheme Bi2WO6/Ta3N5 heterojunctions display dramatically enhanced VLD catalytic activity compared to pristine Bi2WO6, Ta3N5, or the mixture of Bi2WO6 and Ta3N5. Particularly, Bi2WO6/Ta3N5 (1.0Bi–Ta) presents the highest photocatalytic property for the removal of tetracycline hydrochloride (TC) and ciprofloxacin (CIP), achieving approximately 86.7% and 81.1% degradation efficiency, respectively. The extraordinary photocatalytic property is ascribed to the Z-scheme hetero-structure with unique core–shell architecture that realizes compactly interfacial contact between the components for efficient separation of photo-excited carriers, strong visible-light absorption, as well as possesses the strong oxidation ability of photo-excited hole, and the high reduction capacity of photo-excited electron. The trapping experiments combined with electron spin resonance (ESR) analyses verify the prevailing role of photo-induced holes (h+), superoxide radicals (O2−), and hydroxyl radicals (OH) in the Bi2WO6/Ta3N5 photocatalytic system. Notably, the direct contact between Bi2WO6/Ta3N5 and contaminants is experimentally demonstrated to be significant for the efficient degradation of pollutants. Moreover, Bi2WO6/Ta3N5 is endowed with easily recyclable characteristics and excellent durability. Therefore, this research illustrates that Bi2WO6/Ta3N5 may hold a great prospect for the treatment of harmful pharmaceutical pollutants.

Experimental and Theoretical Study into Interface Structure and Band Alignment of the Cu2Zn1-x Cd x SnS4 Heterointerface for Photovoltaic Applications.

To improve the constraints of kesterite Cu2ZnSnS4 (CZTS) solar cell, such as undesirable band alignment at p–n interfaces, bandgap tuning, and fast carrier recombination, cadmium (Cd) is introduced into CZTS nanocrystals forming Cu2Zn1–xCdxSnS4 through cost-effective solution-based method without postannealing or sulfurization treatments. A synergetic experimental–theoretical approach was employed to characterize and assess the optoelectronic properties of Cu2Zn1–xCdxSnS4 materials. Tunable direct band gap energy ranging from 1.51 to 1.03 eV with high absorption coefficient was demonstrated for the Cu2Zn1–xCdxSnS4 nanocrystals with changing Zn/Cd ratio. Such bandgap engineering in Cu2Zn1–xCdxSnS4 helps in effective carrier separation at interface. Ultrafast spectroscopy reveals a longer lifetime and efficient separation of photoexcited charge carriers in Cu2CdSnS4 (CCTS) nanocrystals compared to that of CZTS. We found that there exists a type-II staggered band alignment at the CZTS (CCTS)/CdS interface, from cyclic voltammetric (CV) measurements, corroborated by first-principles density functional theory (DFT) calculations, predicting smaller conduction band offset (CBO) at the CCTS/CdS interface as compared to the CZTS/CdS interface. These results point toward efficient separation of photoexcited carriers across the p–n junction in the ultrafast time scale and highlight a route to improve device performances.

Efficient separation of photoexcited carriers in a g-C3N4-decorated WO3 nanowire array heterojunction as the cathode of a rechargeable Li-O2 battery.

Utilization of solar energy is very important for alleviating the global energy crisis; however, solar-to-electric energy conversion in a compact battery is a great challenge. High charging overpotential of conventional aprotic Li-O2 batteries still restricts their practical application. Herein, we propose a photo-involved rechargeable Li-O2 battery to not only realize direct solar-to-electric energy conversion/storage but also address the overpotential issue. In this photo-involved battery system, the g-C3N4-decorated WO3 nanowire array (WO3@g-C3N4 NWA) heterojunction semiconductor is used as both the photoelectrode and oxygen electrode. Upon charging under visible-light irradiation, the photoexcited holes and electrons are in situ generated on the WO3@g-C3N4 NWA heterojunction cathode. The fabrication of the heterojunction can distinctly reduce the recombination rate between electrons and holes, while photon-generated carriers are effectively and quickly separated and then migrate under a large current density. The discharge product (Li2O2) can be oxidized to O2 and Li+ with a reduced charging voltage (3.69 V) by the abundant photoexcited holes, leading to high energy efficiency, good cycling stability and excellent rate capability. This newly photo-involved reaction scheme could open new avenues toward the design of advanced solar-to-electric energy conversion and storage systems.

Facile synthesis of Bi-modified Nb-doped oxygen defective BiOCl microflowers with enhanced visible-light-driven photocatalytic performance

Bi-modified Nb-doped oxygen defective BiOCl hierarchical microflowers were synthesized through a facile one-pot solvothermal route. The prepared photocatalysts were characterized by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The results indicated that the modified BiOCl appeared as hierarchical microflowers composed of nano sheets. The BiOCl synthesized with Nb/Bi = 1:7 in precursor solutions exhibited the best photocatalytic performance for Rhodamine B (RhB) and Tetracycline (TC) degradation under visible light irradiation. Compared with the BiOCl fabricated through a hydrothermal method, the counterpart with oxygen vacancies (OVs) had a better visible-light-driven photocatalytic activity, and the kinetics constant k for the degradation of RhB increased by 728% (525% for TC). After the modification of Bi0 and Nb5+, the constant k further increased by 134% and 82%, respectively, for the degradation of RhB and TC. This enhanced photocatalytic activity was attributed to both the improved visible light absorption caused by the introduction of OVs and the highly efficient separation of photoexcited carriers resulted from Nb5+ doping and Bi0 deposition. Trapping experiments revealed that the photogenerated hole was the dominating active species. This work provides a new facile strategy for the synthesis of visible-light-driven photocatalysts with high performance.

Field enhanced in-plane homostructure in a pure MoSe2 phototransistor for the efficient separation of photo-excited carriers

A homojunction as a photoresponse enhancement scheme in a pure MoSe2 device is introduced.

Microscopic basis for the band engineering of Mo1-xWxS2-based heterojunction.

Transition-metal dichalcogenide layered materials, consisting of a transition-metal atomic layer sandwiched by two chalcogen atomic layers, have been attracting considerable attention because of their desirable physical properties for semiconductor devices, and a wide variety of pn junctions, which are essential building blocks for electronic and optoelectronic devices, have been realized using these atomically thin structures. Engineering the electronic/optical properties of semiconductors by using such heterojunctions has been a central concept in semiconductor science and technology. Here, we report the first scanning tunneling microscopy/spectroscopy (STM/STS) study on the electronic structures of a monolayer WS2/Mo1−xWxS2 heterojunction that provides a tunable band alignment. The atomically modulated spatial variation in such electronic structures, i.e., a microscopic basis for the band structure of a WS2/Mo1−xWxS2 heterojunction, was directly observed. The macroscopic band structure of Mo1−xWxS2 alloy was well reproduced by the STS spectra averaged over the surface. An electric field of as high as 80 × 106 Vm−1 was observed at the interface for the alloy with x = 0.3, verifying the efficient separation of photoexcited carriers at the interface.

Resonant-tunneling structure of quantum wells in the p-i-n photovoltaic element

The method for efficient separation of photoexcited carriers, based on the resonant tunneling phenomenon in the quantum well structure placed into the i-region of the p-i-n photovoltaic element, is proposed. The parameters of quantum well structures based on the GaAs/Ga1−xInxAs system, implementing the mode of sequential resonant tunneling in the electric field of the GaAs p-i-n junction, is calculated. A microscopic model of resonant-tunneling transport in such structures is constructed, and the kinetic tunneling times are calculated depending on well and barrier parameters. The possibility of achieving sufficiently short (<∼10 ps) tunneling times and, hence, quite efficient removal of photoelectrons and photoholes from quantum wells at a proper choice of barrier powers is shown.

Intensity modulated photocurrent spectroscopy (IMPS) of solid-state polybithiophene-based solar cells

Intensity modulated photocurrent spectroscopy (IMPS) was used to study the mechanism and kinetics of carrier extraction at contacts in polybithiophene-based solid-state photovoltaic cells. The cells were prepared using traditional architecture with ITO/PEDOT and Al contacts. It was shown that such an architecture does not provide a sufficient built-in voltage for the efficient separation of photoexcited carriers without applying considerable external bias. Furthermore, even when such a bias was applied, the resulting dc photocurrents were still very low due to a high rate of recombination at the contacts. The results highlight the importance of improving the efficiency of carrier extraction at contacts and the cell architecture in general if the overall efficiency of polymer-based solar cells is to be further increased.