Semiconductor quantum dots: Technological progress and future challenges.

As closely related to exciton quenching, Auger recombination (AR) is very important to colloidal semiconductor quantum dots (QDs) in applications such as lasing, solar energy conversion, and light‐emitting diodes. It is beneficial to understand the kinetic mechanism of AR. In this article, colloidal CdSe QDs with a narrow range of diameters, 2.7, 2.5, 2.3, 2.2, and 2.0 nm, are synthesized by adequately modulating the reaction temperature and time during hot injection synthesis. With sizes less than the Bohr exciton radius, the resultant collision probability differs for various CdSe QDs attributed to the varying extent of the spatial confinement effect. To analyze the bi‐exciton AR in these QDs, the ultrafast time‐resolved absorption spectrum is utilized to derive the bi‐exciton lifetime (τBX) in high precision as a function of QD size. The τBX varying with volume (V) obeys the universal scaling law; that is, τBX = γV. In our experiments, the scaling factor γ is 1.649 ± 0.097 ps/nm3. This study could provide further insight for understanding the theory of the AR process and rational design of optoelectronic devices based on colloidal QDs.

The distinctive chemical, physical, electrical, and optical properties of semiconductor quantum dots (QDs) make them a highly fascinating nanomaterial that has been extensively studied. The CuInSe2 (CIS) QDs demonstrates great potential as a nontoxic alternative to CdSe and PbSe QDs for realizing high-performance solution-processed semiconductor devices. The CIS QDs show strong light absorption and bright emission across the visible and infrared spectrum and have been designed to exhibit optical gain. The special characteristics of these properties are of great significance in the fields of solar energy conversion, display, and electronic devices. Here, we present a comprehensive overview of the potential applications of colloidal CIS QDs in various fields, with a particular focus on solar energy conversion (such as QD solar cells, QD-sensitized solar cells, and QD luminescence solar concentrators), solar-to-hydrogen production (such as photocatalytic and photoelectrochemical H2 production), and QD electronics (such as QD transistors, QD light-emitting diodes, and QD photodetectors). Furthermore, we offer our insights into the current challenges and future opportunities associated with CIS QDs for further research.

Open AccessCCS ChemistryRESEARCH ARTICLES1 Sep 2023Osmotic-Enhanced-Photoelectrochemical Hydrogen Production Based on Nanofluidics Pei Liu†, Xiao-Ya Gao†, Li-Jun Zhang, Weipeng Chen, Yuhao Hu, Xiang-Yu Kong, Xu-Bing Li, Liping Wen, Chen-Ho Tung, Li-Zhu Wu and Lei Jiang Pei Liu† Key Laboratory of Bio-inspired Materials and Interfacial Science, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 , Xiao-Ya Gao† Key Laboratory of Bio-inspired Materials and Interfacial Science, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 , Li-Jun Zhang Key Laboratory of Bio-inspired Materials and Interfacial Science, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 , Weipeng Chen Key Laboratory of Bio-inspired Materials and Interfacial Science, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 , Yuhao Hu Key Laboratory of Bio-inspired Materials and Interfacial Science, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 , Xiang-Yu Kong *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Bio-inspired Materials and Interfacial Science, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 , Xu-Bing Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Bio-inspired Materials and Interfacial Science, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 , Liping Wen *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Bio-inspired Materials and Interfacial Science, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 , Chen-Ho Tung Key Laboratory of Bio-inspired Materials and Interfacial Science, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 , Li-Zhu Wu *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Bio-inspired Materials and Interfacial Science, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 and Lei Jiang *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] Key Laboratory of Bio-inspired Materials and Interfacial Science, Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190 School of Future Technology, University of Chinese Academy of Sciences, Beijing 100049 https://doi.org/10.31635/ccschem.022.202202215 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Harvesting clean energy such as solar energy and salinity gradient energy directly from the surrounding environment has attracted great attention. A promising proof-of-concept combination of cation-selective membrane-based osmotic energy with photoelectrochemical-based solar energy has been developed, highlighting the great potential for the direct conversion of osmotic energy to hydrogen energy. With the help of a 50-fold concentration gradient, the MXene–CdSe quantum dots system exhibits the highest photocurrent enhancement ratio (ΔIL-H/ΔIL-L), and the hydrogen production is increased by about 33% at a bias of 0 V versus reversible hydrogen electrode. Directly converting osmotic energy and solar energy into hydrogen energy suggests the possibility of coupling osmotic energy with other renewable energy sources. Download figure Download PowerPoint Introduction The rapid consumption of fossil fuels and the ensuing environmental problems have forced the exploration of low-cost, sustainable, and ecofriendly energy sources.1–3 Renewable energy generated from solar, wind, biomass, hydro, ocean, and heat in the earth's crust has gained increasing attention.3–5 Ocean energy has become an important part of the development of new energy due to its large storage capacity and diverse development methods; particularly, osmotic energy from the salinity gradient between ocean water and river water, also known as blue energy, has recently been identified as a promising source of clean energy.6–9 Nanofluidics combine strong confinement and surface-charge effects at the nanoscale, which contribute to novel ion transport properties including excellent selectivity and high flux.10 Nanofluidic channels with tailored ion transport dynamics enable high-performance reverse electrodialysis to efficiently harvest renewable osmotic energy. Therefore, various symmetric and asymmetric nanofluidics using materials of different dimensions have been developed to finely control ion transport behaviors to capture osmotic energy.11–14 However, almost all osmotic energy is converted into electrical energy and then for further power output or external resistance supply (see Supporting Information Figure S1). Solar energy transformation via photoelectrochemical (PEC) hydrogen generation is another important pathway of clean energy generation.15–19 With the excitation of suitable visible light, hydrogen production by photogenerated electrons and oxygen generation by photogenerated holes can be realized with a rationally designed PEC system, which can convert solar energy into chemical energy. However, there are few studies reported on enhancing PEC hydrogen generation with the help of osmotic energy.11 In contrast, PEC systems demand an additional bias voltage to assist directional transmission of charge and enhance carrier separation,17,20,21 whereas osmotic energy and internal concentration gradient can generate an extra potential. The introduction of osmotic energy might contribute to the enhancement of PEC water splitting. Additionally, the membrane separates the electrolyte between anode and cathode to avoid the side reaction of products and separately collects the products generated on the electrode surface. Therefore, it would be inspiring for simultaneously storing solar energy and osmotic energy in chemical bonds. With this in mind, we embarked on the design of a system that could directly convert osmotic energy and solar energy to chemical products, such as H2 and O2 (Figure 1). Among various materials, semiconductor quantum dots (QDs) have been widely used for artificial photosynthetic hydrogen production because of their unique properties such as large extinction coefficients, abundant surface sites, and tunable band position. Moreover, QDs can evolve H2 gas at the exposed metal sites in the absence of external cocatalysts. Therefore, we used colloidal QDs in this system.22 Under light irradiation, the QDs sensitized photocathode would absorb photons and generate the photoinduced electron-hole pairs. The photogenerated holes transfer from the valence band of QDs to the counter electrode (Pt) to participate in the oxidation reactions, and the photogenerated electrons could migrate to the electrode surface followed by proton reduction, that is, H2 generation. With the participation of the concentration gradient and the cation selectivity membrane, the directional migration of Na+ across the membrane would occur, thus leading to the accumulation of cations on the photocathode side. To maintain the electrical neutrality, cations need to be consumed and the reduction of protons on the photocathode would be facilitated,23 thereby achieving osmotic-enhanced PEC hydrogen production. In brief, this process could directly convert osmotic energy into hydrogen energy and further promote hydrogen production. Figure 1 | Coupling of osmotic-energy conversion and the PEC hydrogen production. Under light irradiation, the QD-sensitized photocathode would absorb photons and generate the photoinduced charge carriers, electron–hole pairs. The holes transfer from the QDs to the counter electrode (Pt) through an external circuit and oxidize water to generate O2 while the remaining electrons are used to reduce protons into H2, simultaneously. Meanwhile, directional migration of Na+ across the cation selective membrane driven by concentration difference leads to the accumulation of cations in the low-concentration side. To maintain electrical neutrality, the reduction of protons on the photocathode is enhanced, thereby achieving osmotic-enhanced PEC hydrogen production. This system realizes the direct conversion of solar energy and osmotic energy to hydrogen energy. Download figure Download PowerPoint Here, we provide a demonstration of osmotic enhanced PEC hydrogen production based on nanofluidics (Figure 1). Osmotic energy is introduced into the PEC hydrogen production system by using various cation selective membranes made of materials with different dimensions. Upon visible-light irradiation, the photocurrent of the PEC system with the concentration gradient is significantly enhanced compared with that without the concentration gradient. The MXene–CdSe QDs system shows the highest photocurrent enhancement ratio (ΔIL-H/ΔIL-L) and realizes an approximately 33% increase in hydrogen production at a bias of 0 V versus reversible hydrogen electrode (RHE) while maintaining a stable Faraday efficiency, which does not require any sacrificial agents or external cocatalyst. The intended system proposes and verifies the concept of osmotic-enhanced PEC hydrogen production for the first time. Furthermore, this work has realized the direct conversion of osmotic energy to other renewable energy sources such as hydrogen energy. Experimental Methods Fabrication of cellulose nanofiber, MXene, acrylic acid, graphene oxide, and molybdenum disulfide membrane Cellulose nanofiber membrane 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofiber (CNF) gel (1 wt %) was purchased from Woodelfbio Cellulose Co., Ltd. (Tianjin, China). A certain quantity of CNF gel was dispersed in deionized (DI) water by ultrasound to obtain a dispersion of 1 mg/mL; then, 30 mL CNF dispersion was filtered on polycarbonate (PC, pore size ∼200 nm) and self-assembled into a free-standing membrane. Finally, the CNF membrane was peeled off easily after drying in air. MXene membrane First, 0.66 g LiF was slowly added to 10 mL of 9 M HCl solution and stirred for 5 min. Then, 1 g MAX Ti3AlC2 powders (98%, 200 mesh, purchased from Forsman Scientific Co., Ltd., Beijing, China) were added and stirred at 800 rpm for 24 h at 35 °C. After etching, the mixture was centrifuged and washed with DI water at 3500 r.p.m. for 5 min each until a pH ∼6. The settled powders were dried for at least 12 h and collected. A certain quantity of Ti3C2Tx was dissolved into dimethyl sulfoxide and then magnetically stirred at room temperature for 2 h to delaminate. The intercalated-Ti3C2Tx was then dispersed in DI water with concentration of 1 mg/mL, followed by sonication under flowing argon for 10 min, and then centrifugation for 1 h at 3500 rpm. The MXene supernatant was obtained, and then 500 μL MXene dispersion was filtered on anodic aluminum oxide (AAO, pore size 110–150 nm) to obtain the 2D MXene membrane. Acrylic acid membrane The precursor solution for preparing acrylic acid (AAc) hydrogel membrane was composed of 2 g of AAc, 40 mg of N,N′-methylenebis(acrylamide), 0.1 g of methyl methacrylate, 10 μL of 2,2-diethoxyacetophenone, and 7.9 g of DI water. First, the precursor solution was poured on PC film for 5 min to ensure the solution penetrated the channel and filled the space. The excess precursor solution was removed by filter paper. After the photopolymerization under UV irradiation (PL-LED100, 365 nm, Perfectlight Technology Co., Ltd., Beijing, China), the hybrid membrane was obtained. Before testing, all the hydrogel hybrid membranes were soaked in water for 24 h (with a fresh water change every 6 h) to ensure the removal of excess monomers and stored in water for later use. Graphene oxide and molybdenum disulfide membrane Graphene oxide (GO) and molybdenum disulfide (MoS2) dispersions were purchased from XFNANOMaterials Tech Co., Ltd. (Nanjing, China). The GO or MoS2 dispersions (500 μL) were filtered on an AAO (pore size 110–150 nm) to obtain the 2D membrane. Fabrication of QDs/NiO/fluorine-doped tin oxide electrode Fluorine-doped tin oxide/NiO electrode The NiO precursor solution was first prepared by reflux of 6 mmol of Ni(Ac)2·4H2O, 6 mmol of ethanolamine, and 20 mL of 2-methoxyethanol at 70 °C for 1 h. Fluorine-doped tin oxide (FTO) glasses (2 cm × 2 cm) were sonicated respectively for 30 min in detergent solution, water, and ethanol to remove contaminant. The NiO precursor solution (180 μL) was printed on the precleaned FTO glass using the spin-coating method (3000 r.p.m., 30 s), followed by being sintered at 400 °C for 60 min in a muffle furnace with the heating rate of 8 °C/min. Finally, FTO/NiO electrode was obtained by natural cooling. QDs/NiO/FTO electrode QDs (5 mL) was precipitated by adding 25 μL of HCl (1 M), followed by centrifuging and washing with water twice. The precipitate was dispersed in 800 μL of water and sonicated for 30 min. The dispersed solution (160 μL) was dropped on the surface (1 cm × 1 cm) of the FTO/NiO electrode (2 cm × 1 cm), and the electrode was dried naturally. Finally, the electrode was calcined in a tube furnace under argon atmosphere for 20 min at 150 °C with the heating rate of 5 °C/min. Electrical measurements The ionic transport properties and osmotic energy conversion tests of cation selectivity membranes were performed with a Keithley 6487 semiconductor picoammeter (Keithley Instruments, Cleveland, OH). The membranes were mounted between a two-compartment conductivity cell that contained about 3.0 mL testing solution (see Supporting Information Figure S2). For the ionic transport measurement, KCl solutions from 0.1 μM to 1 M were used to fill two chambers. Homemade Ag/AgCl electrodes were used to apply a transmembrane potential across the membrane. For the energy conversion measurement, concentrated and diluted solutions were filled into two sides of the membrane. Homemade Ag/AgCl electrodes were used to collect the osmotic energy. The short-circuit current ISC (the current corresponding to zero external bias) and open-circuit potential VOC (the potential corresponding to the zero current) were obtained from the I–V measurements. In these measurements, the effective experimental area was about 3.14 mm2. All the measurement were conducted under pH 6. PEC H2 evolution The schematic diagram of the PEC device is shown in Figure 5a. The cation selective membrane is fixed in the middle of the two cells. The prepared electrode acted as the working electrode, Ag/AgCl (saturated KCl) as the reference electrode, and platinum as the counter electrode. Na2SO4 aqueous solution (80 mL) built the concentration gradient and acted as the electrolyte. After sealing, the electrolyte solution was bubbled with inert argon gas for 20 min to remove oxygen. The I–t measurement was tested by an electrochemical workstation with blue light-emitting diodes (LEDs) irradiating the working electrode. The produced hydrogen was detected by gas chromatograph (GC) with CH4 as an internal standard. In the isotope labeling experiments, the water on the counter electrode side was replaced with 1:1 H216O and H218O, with other conditions unchanged. 18O2 was detected by gas chromatography-mass spectrometry (GC-MS). All light irradiation experiments were carried out at room temperature with ambient temperature condensate. Results and Discussion One-dimensional (1D) nanofibers (CNF), two-dimensional (2D) nanosheets (MXene), and three-dimensional (3D) hydrogels (AAc) were selected to build cation selective membranes (Figure 2a).24–26 The length and diameter of the CNF were in the range of micrometer and nanometer scale, respectively (see Supporting Information Figure S3), which rendered the decent stacking of the 1D nanofibers to form the nanofluidic membrane (Figure 2a-i). Ti3AlC2 was chosen to fabricate MXene nanosheets as reported in our previous work.25,27 The obtained 2D MXene nanosheets could be reassembled through vacuum assisted filtration to get a flexible thin membrane attached on the AAO substrate whose channel size was within the range of 110–150 nm. The cross-sectional scanning electron microscopy (SEM, Supporting Information Note 3) image of the MXene membrane showed an ordered lamellar structure (Fig. 2a-ii). 3D hydrogel networks were constructed through radical initiated polymerization in the channels of the PC film. As shown in Fig. 2a-iii, the channels of the PC film were fully filled with 3D hydrogel after hybridization. Moreover, X-ray photoelectron spectroscopy (XPS) was used to further study the functional groups of the three membranes (see Supporting Information Figure S4). The cation selectivity of CNF and AAc membranes benefited from the carboxyl groups (–COOH). Furthermore, in the high-resolution XPS spectrum of the MXene membrane, the O1s spectrum indicated four components: O–Ti, O–Ti/OH, O–C/OH, and H2O. These surface groups imparted the MXene nanosheets a negatively charged surface and cation selectivity. Figure 2 | Characterization and osmotic-energy conversion behavior of the three cation-selective membranes made from materials of different dimensions: 1D CNF, 2D MXene, and 3D AAc hydrogels. (a) Cross-sectional SEM and the corresponding schematics of the structure of the three membranes: (i) 1D CNF, (ii) 2D MXene, and (iii) 3D AAc hydrogels. (b) Ion conductance of the membranes versus KCl concentration, showing a typical charge-governed ion transport behavior. (c) ISC and (d) VOC under different concentration gradient (NaCl). The low-concentration side is fixed at 0.01 M. Download figure Download PowerPoint The ionic transport properties of the three cation selectivity membranes were characterized under various ionic concentrations. Supporting Information Figure S5a shows the I–V characteristics of various membranes under symmetric electrolyte condition (0.01 M KCl, pH ∼7). The MXene membrane exhibited the smallest current value at −2 V among the three materials (See Supporting Information Figure S5b) due to its sub-nanometer-sized ion channels. Moreover, transmembrane ionic conductance was significantly affected by electrolyte concentration (Figure 2b). These conductance curves gradually deviated from the volume value (gray dotted line) and showed saturation for low salt concentration, indicating the typical surface-charge-controlled ion transport process across the membrane.28 Next, the osmotic energy conversion performance was demonstrated by using NaCl with various concentration gradients applied across the composite membrane. The short-circuit current (ISC, current at 0 V) and open-circuit potential (VOC, potential corresponding to zero current) are summarized in Figure 2c,d. As mentioned above, the ion channels of the MXene membrane were sub-nanometer-sized so that the corresponding ISC and VOC were lower than those of CNF and AAc. The high cation selectivity of the AAc membrane owing to the space charge from the carboxylic group of the hydrogel 3D network led to the higher osmotic energy conversion performance.24,29 Then, osmotic enhanced PEC hydrogen production was investigated based on the above three cation selectivity membranes. First, the PEC hydrogen generation system was constructed with a sensitized photocathode (FTO/NiO/QDs), QDs as the photocatalyst unit, and p-type NiO as the hole transfer layer. Three kinds of QDs were selected to fabricate the photocathodes in this process ( Supporting Information Note 1).30–32 Dynamic light scattering ( Supporting Information Figure S6) indicated the stable colloidal property and good dispersion of mercaptopropionic acid (MPA) stabilized CdSe QDs, CdS QDs, and CdSe/CdS core-shell QDs in water. Figure 3a displays the high-resolution transmission electron microscopy (TEM) images of CdSe QDs, CdS QDs, and CdSe/CdS core-shell QDs. Upon light irradiation, the QDs sensitized photocathode would generate the photoinduced electron-hole pairs. The photoinduced holes transferred from QDs to the counter electrode (Pt) through an external circuit and oxidized water to generate O2 while the electrons were involved in simultaneous hydrogen production. UV–vis absorption spectra of CdSe, CdS, and CdSe/CdS QDs are shown in Figure 3b, corresponding to the first exciton absorption peak of 438, 420, and 468 nm, respectively, indicating their good visible-light response. Figure 3 | Osmotic-enhanced photocurrent responses to on–off illumination. (a) Schematic illustration of the photocathode and the migration of photogenerated excitons at the interfaces. The corresponding high-resolution TEM images of CdSe QDs, CdS QDs, and CdSe/CdS QDs are shown on the right. (b) UV–vis spectra and photographs of the CdSe QDs, CdS QDs, and CdSe/CdS QDs. (c) The difference between the photocurrent and the dark-current (IPhoto − IDark) with (L-H: 0.01 M/0.5 M Na2SO4) or without concentration gradient (L-L: 0.01 M/0.01 M Na2SO4) at a bias of 0 V versus RHE. (d) Photocurrent enhancement ratio (ΔIL-H/ΔIL-L) of different systems. Download figure Download PowerPoint For concept study, osmotic-enhanced PEC hydrogen generation performance was investigated in a three-electrode H-shape cell separated by cation selective membranes with QDs-sensitized photocathode as the working electrode, platinum (Pt) as the counter electrode, and Ag/AgCl as the reference electrode ( Supporting Information Figure S7, see details in SI). Oxygen-free Na2SO4 aqueous solution with certain concentration acted as the electrolyte. Upon the same or different electrolyte concentration for sides of the membrane, the initiated a photocurrent with a bias of 0 V versus under blue nm) irradiation, hole transfer to counter electrode and proton reduction on the surface of the photocathode ( Supporting Information The introduction of concentration gradient and cation selectivity membrane led to the accumulation of cations on the photocathode side and further the consumption of achieving an enhancement of photocurrent ( Supporting Information that the introduction of working as a on the enhancement of photocurrent in the composite membrane ( Supporting Information Figure Additionally, the difference between photocurrent and dark-current (IPhoto − IDark) with (L-H: 0.01 M/0.5 M Na2SO4) or without concentration gradient (L-L: 0.01 M/0.01 M Na2SO4) are summarized in Figure a bias of 0 V The − under the 50-fold concentration gradient was significantly higher than that without concentration gradient for all systems. a range of cation selective membranes CNF, MXene, and and three kinds of QDs CdS, and CdSe/CdS the photocurrent enhancement ratio (ΔIL-H/ΔIL-L) was realized by osmotic with MXene as the cation-selective membrane, and CdSe QDs as the photocatalyst (Figure with composed of 1D and 3D materials, membranes constructed from materials ordered ion transport and thus have lower ion transport Moreover, different cation selective membranes provide a different due to different cation ion and ion migration and these could also to different of within the membrane. The photocurrent enhanced for the system might be to the between the of within the membrane and the of the The was ( Supporting Information Figure and the for the system suggests it is the with the condition without the concentration gradient, the photocurrent in the of concentration gradient exhibits a increase with the of the potential because the salinity difference between and gradually thus leading to the lower open-circuit Furthermore, compared with conditions the concentration gradient, exhibits a and PEC within the potential which that the concentration gradient could provide potential to increase PEC water splitting. to the different selective membranes different PEC with the same CdSe QDs Based on of the of electrons transferred by the chemical reaction that at the photocathode or is to the quantity of through the The PEC water reaction is driven by the the extra and the osmotic potential by the concentration gradient. With the same and the reaction condition concentration of the leading to the different reaction is the different osmotic energy, which leads to different of within the membrane and a different Furthermore, to the of the osmotic enhanced PEC hydrogen additional experiments were performed with two other 2D nanofluidic systems based on MoS2 and The obtained 2D dispersion ( Supporting Information Figure could be reassembled by vacuum filtration on the AAO to get the flexible thin membrane ( Supporting Information Figure The strong peak in the X-ray (Figure suggests 2D membranes. cation selectivity can be by the potential ( Supporting Information

Smart functional fiber devices are indispensable for optical fiber based communication and sensing systems, in which the integration of fiber structure with smart semiconductor materials is essential. Among various choices, the colloidal quantum dot (CQD) is of great interests and its advantages includes: (a) the concentration of the photo-excited electrons can be relatively high due to the quantum confinement in the CQD, which means the refractive index of CQD could be altered with less power consumption; (b) CQD is a nanoscale dispersion solution thus the functional material can be deposited conformally on fiber, silicon chips or other complex structure surface; (c) CQD is a highly tunable material, and the chemical composition and operation wavelength can be specially designed for any wavelength window. In this work, we developed a platform to deposit the CQD with fiber structures for smart sensing and communication applications. First, we construct optical fiber antennas for gas sensing, in which the CQD conformally coated on the fiber surface behave like olfactory receptors. The gas molecules adsorbed on the quantum dots change the local carrier concentration in quantum dot solids, which leads to a change in their refractive index. This interaction of gas molecules with quantum dots could be transformed into optical signals through the optical fiber antennae. Owing to the large surface area, highly tunable physical and chemical properties of colloidal quantum dots, combined with the versatile fiber microstructure, the optical fiber antennae with quantum dots offer a new degree of freedom to precise, real-time and large-scale gas monitoring. Second, by using the light-excited carriers in CQD, a light controlled fiber device has been achieved as a delayed interferometric comb filter. The transmission spectrum can be easily tailored by mW-scale optical power. This device was applied in a 50 km standard single mode fiber (SSMF) based communication system with 12.5 Gbs On-Off-Keying (OOK) direct modulation for chirp management and dispersion compensation to achieve the error-free transmission.

Rational design of colloidal AgGaS2/CdSeS core/shell quantum dots for solar energy conversion and light detection

Control of composition, stoichiometry, and defects in colloidal quantum dots (QDs) of III-V semiconductors has proven to be difficult due to their covalent character. Whereas the synthesis of colloidal indium pnictides such as InP, InAs, and InSb has made significant progress, gallium-containing colloidal III-V QDs still remain largely elusive. Gallium pnictides represent an important class of semiconductors due to their excellent optoelectronic properties in the bulk; however, the difficulty with the synthesis of gallium-containing colloidal III-V QDs has largely prohibited their exploration as solution-processed semiconductors. Here we introduce molten inorganic salts as high-temperature solvents for the synthesis and manipulation of III-V QDs. We demonstrate cation exchange reactions on presynthesized InP and InAs QDs to form In1- xGa xP and In1- xGa xAs QDs at temperatures above 380 °C. This approach produces novel ternary alloy QDs with controllable compositions that show size- and composition-dependent absorption and emission features. Emission quantum yields of up to ∼50% can be obtained for In1- xGa xP/ZnS core-shell QDs. A comparison of the optical properties of InP/ZnS core-shells with In1- xGa xP/ZnS core-shells reveals that Ga incorporation leads to significant improvement in the optical properties of III-V/II-VI core-shell emitters which is of great importance for quantum dot-based lighting and display applications. This work also demonstrates the potential of molten inorganic salts as versatile solvents for the synthesis and processing of colloidal nanomaterials at temperatures inaccessible for traditional solvents.

Structure of a subnanometer-sized semiconductor Cd14Se13 cluster

Semiconductor quantum dots (QDs) are very interesting candidates for the development of green-energy based nanomaterials and devices. However, currently they present serious concerns because the large majority of efficient QDs have been synthesized using organometallic routes with toxic heavy metals. Herein, we designed and produced novel fluorescent ternary and quaternary QD nanostructures based on AgInS2 (AIS) core and ZnS (ZAIS) shell stabilized with carboxymethyl cellulose (CMC) polymer ligand for potential applications in green solar energy harvesting. Colloidal AgInS2 QDs were prepared by co-precipitation process using a one-pot aqueous green route directly stabilized by CMC at room temperature and varying pH conditions. Then, an outlayer of ZnS was grown and thermally annealed to improve and tune their optical properties and split the emission range, leading to core–shell alloyed nanostructures. Their physicochemical and optical properties were extensively characterized, demonstrating that photoluminescent monodispersed AIS and ZAIS QDs were produced with size typically ranging from 8.5 ± 2.4 nm and 3.2 ± 1.1 nm under acidic or alkaline media, respectively. Therefore, by adjusting the parameters of synthesis, the QD nanohybrids showed tunable optical properties ranging from UV to NIR, with maxima absorption/emission in the visible-range of the light spectrum (from λ = 650 to 750 nm). More importantly, the optical properties demonstrated remarkable enhancement of quantum yield of over one order of magnitude due to the combination of processing parameters and engineered core–shell nano-architecture of AgInS2–ZnS. Thus, these luminescent nanomaterials offer great perspectives for developing innovative broad spectral converters for a wide range of applications in solar energy harvesting.

In quantum-confined semiconductor nanostructures, electrons show unique behaviors compared to bulk solids, which endows materials with tunable physicochemical and photoelectric properties. Zero-dimensional semiconductor quantum dots (QDs) provide intense light absorption and strong narrowband emission at visible and infrared bands, and have been employed to exhibit optical gain and lasing. These properties are beneficial for different application. Here, we provide a review in the synthesis and understanding of unique properties of colloidal QDs, and discuss their applications in display, lasers, sensing and solar energy conversion.

Colloidal quantum dots (CQDs) are considered as next-generation semiconductors owing to their tunable optical and electrical properties depending on their particle size and shape. The characteristics of CQDs are mainly governed by their surface chemistry, and the ligand exchange process plays a crucial role in determining their surface states. Worldwide studies toward the realization of high-quality quantum dots have led to advances in ligand exchange methods, and these procedures are usually carried out in either solid-state or solution-phase. In this article, we review recent advances in solid-state and solution-phase ligand exchange processes that enhance the performance and stability of lead sulfide (PbS) CQD solar cells, including infrared (IR) CQD photovoltaics.

Hypophosphite tailored graphitized hierarchical porous biochar toward highly efficient solar thermal energy harvesting and stable Storage/Release

A novel periodic array CdSe nanowire is prepared on a substrate of the porous titanium dioxide by using a self-assembly method of the colloidal CdSe quantum dots (QDs). The experimental results show that the colloidal CdSe QDs have renewedly assembled on its space scale and direction in process of losing background solvent and form the periodic array nanowire. The main peak wavelength of Photoluminescence (PL) spectra, which is measured by using a 100-nm aperture laser beam spot on a scanning near-field optics microscopy, has shifted 60 nm with compared to the colloidal CdSe QDs. Furthermore, we have measured smaller ordered nanometer structure in thin QDs area as well, a 343-nm periodic nanowire in thick QDs area and the colloidal QDs in edge of well-ordered nanowire.

ABSTRACTExchanging the original organic ligands of colloidal CdSe quantum dots (QDs) with metal chalcogenide SnS4 ligands resulted in absorption peak redshifts and complete photoluminescence quenching in QD solids. The ITO/QDs/Al structure with SnS4-capped QDs showed much higher electrical conductivity and reduced space-charge limited current. These results are indicative of carrier delocalization as well as enhanced inter-QD electronic coupling caused by the inorganic ligands. The SnS4-capped QDs were able to retain strong excitonic absorption. The photocurrent spectral response of the all-inorganic QD film resembled its absorption spectra, and was three orders of magnitude stronger than that of QDs with organic ligands. It was found that mild annealing at ∼ 200 oC transformed the SnS4-capped QD film into to a more conductive assembly, degrading its absorption and photocurrent generation. These findings suggest that colloidal QDs with metal chalcogenide ligands are better suited for solar energy conversion and photodetection than use in light-emitting devices as luminophores.

Colloidal quantum dots (QDs) have garnered significant attention for their exceptional optoelectronic properties and excellent chemical processability, making them highly promising for a wide range of applications. To unlock their full potential, synthesizing QDs with high crystal quality and stability is crucial, as surface defects act as non-radiative recombination centers, leading to luminescence quenching and reduced device performance.In recent years, we have successfully synthesized low-defect QDs, including PbS QDs and perovskite QDs such as APbX₃ (A = FA, MA, Cs; X = I, Br, Cl) QDs, Sn-Pb alloyed QDs, and Sn-based QDs [1–11]. These advances have significantly improved the photophysical properties and stability of QDs, laying a strong foundation for enhanced device performance. PbS QDs, in particular, exhibit excellent infrared absorption, size-tunable bandgaps, and high defect tolerance, making them ideal for optoelectronic applications. Perovskite QDs have also demonstrated high quantum yields and tunable optical properties, further driving research interest.QD-based solar cells (QDSCs) are promising next-generation photovoltaics due to their low cost and potential to surpass the Shockley–Queisser limit through multi-exciton generation and hot carrier extraction. However, the current power conversion efficiency (PCE) of QDSCs remains well below the theoretical limit, primarily due to inefficient charge transport and high non-radiative recombination at the interfaces. Interface defects and charge trapping sites at grain boundaries significantly limit carrier diffusion and extraction. To address these issues, we developed advanced interface engineering strategies, including surface passivation and ligand exchange, which have successfully minimized defect states and improved charge transfer efficiency. Recently, we achieved over 15% PCE for PbS QDSCs, which is the highest reported PCE for PbS QDSCs to date [11].A key focus of our research is on hot carrier dynamics and their extraction from perovskite quantum dots (PQDs) — a critical challenge in next-generation optoelectronic devices. Hot carriers, generated by high-energy photon absorption, rapidly lose their excess energy through carrier cooling before they can be harvested. This cooling process, occurring on a sub-picosecond timescale, severely limits the potential for hot carrier extraction and utilization. We systematically investigated the photoexcited hot and cold carrier dynamics and charge transfer at the heterojunction [8-10]. We found that hot carriers are difficult to extract from PQDs into conventional metal oxides such as TiO₂ and NiO₂ due to rapid carrier cooling [8]. To address this challenge, we developed a PQD–fullerene hybrid structure that enhances carrier coupling at the heterojunction and suppresses carrier cooling. This approach enabled a remarkable hot carrier capture efficiency of 76% [10]. Fullerene acts as an efficient electron acceptor, creating a strong interfacial electronic coupling between PQDs and fullerene that facilitates rapid hot carrier extraction before thermalization occurs.In this talk, I will present recent breakthroughs in improving QDSC performance through interface engineering, charge extraction optimization, and hot carrier management in PQDs. Strategies to reduce non-radiative recombination, improve charge extraction, and optimize device architecture will be discussed, along with insights into hot carrier cooling and extraction dynamics of perovskite QDs. These advances represent a significant step toward achieving high-performance QDSCs with efficiencies approaching their theoretical limits. References F. Liu, Q. Shen et al., ACS Nano 11 (2017) 10373.F. Liu, Q. Shen et al., J. Am. Chem. Soc. 139 (2017) 16708.F. Liu, Q. Shen et al., Chem. Mater. 32 (2020) 1089.F. Liu, Q. Shen et al., Angew. Chem. Int. Ed. 59 (2020) 8421.J. Jiang, F. Liu, Q. Shen, S.X. Tao, J. Mater. Chem. A 9 (2021) 12087.F. Liu, Q. Shen et al., ACS Appl. Nano Mater. 4 (2021) 3958.Y. Li, Q. Shen et al., J. Am. Chem. Soc. 139 (2024) 16708.C. Ding, Q. Shen et al., Nano Energy 67 (2020) 104267.H. Li, Q. Shen et al., Adv. Mater. 35 (2023) 2301834.Y. Li, Q. Shen et al., Adv. Funct. Mater. 35 (2025) 2415735.D. Chao, Q. Shen et al., Adv. Energy Mater. 12 (2022) 2201676.

"Green" colloidal quantum dots (QDs)-based photoelectrochemical (PEC) cells are promising solar energy conversion systems possessing environmental friendliness, cost-effectiveness, and highly efficient solar-to-hydrogen conversion. In this work, eco-friendly AgInSe (AISe)/ZnSe core/shell QDs with wurtzite (WZ) phase were synthesized for solar hydrogen production. It was demonstrated that appropriately engineering the ZnSe shell thickness resulted in effective surface defects passivation of the AISe core for suppressed charge recombination in the consequent core/shell AISe/ZnSe QDs. The fabricated environmentally friendly core/shell QDs-based PEC device exhibited improved photo-excited electrons extraction efficiency under optimized conditions and delivered a maximum photocurrent density as high as 7.5 mA cm-2 and long-term durability under standard AM 1.5G illumination (100 mW cm-2 ). These findings suggest that AISe/ZnSe core/shell QDs with tailored optoelectronic properties are potential light sensitizers for eco-friendly, cost-effective, and highly efficient solar energy conversion applications.