Surface Stoichiometry Control of Colloidal Heterostructured Quantum Dots for High-Performance Photoelectrochemical Hydrogen Generation

Manipulating the separation and transfer behaviors of charges has long been pursued for promoting the photoelectrochemical (PEC) hydrogen generation based on II–VI quantum dot (QDs), but remains challenging due to the lack of effective strategies. Herein, a facile strategy is reported to regulate the recombination and transfer of interfacial charges through tuning the surface stoichiometry of heterostructured QDs. Using this method, it is demonstrated that the PEC cells based on CdSe-(Se_xS_1−x)₄-(CdS)₂ core/shell QDs with a proper S_surface/Cd_surface ratio exhibits a remarkably improved photocurrent density (≈18.4 mA cm⁻² under one sun illumination), superior to the PEC cells based on QDs with Cd-rich or excessive S-rich surface. In-depth electrochemical and spectroscopic characterizations reveal the critical role (hole traps) of surface S atoms in suppressing the recombination of photogenerated charges, and further attribute the inferior performance of excessive S-rich QDs to the impeded charge transfer from QDs to TiO₂ and electrolyte. This work puts forward a simple surface engineering strategy for improving the performance of QDs PEC cells, providing an efficient method to guide the surface design of QDs for their applications in other optoelectronic devices.     

Effect of Surface Coating on the Photocatalytic Function of Hybrid CdS–Au Nanorods

Hybrid semiconductor–metal nanoparticles are interesting materials for use as photocatalysts due to their tunable properties and chemical processibility. Their function in the evolution of hydrogen in photocatalytic water splitting is the subject of intense current investigation. Here, the effects of the surface coatings on the photocatalytic function are studied, with Au‐tipped CdS nanorods as a model hybrid nanoparticle system. Kinetic measurements of the hydrogen evolution rate following photocatalytic water reduction are performed on similar nanoparticles but with different surface coatings, including various types of thiolated alkyl ligands and different polymer coatings. The apparent hydrogen evolution quantum yields are found to strongly depend on the surface coating. The lowest yields are observed for thiolated alkyl ligands. Intermediate values are obtained with L‐glutathione and poly(styrene‐co‐maleic anhydride) polymer coatings. The highest efficiency is obtained for polyethylenimine (PEI) polymer coating. These pronounced differences in the photocatalytic efficiencies are correlated with ultrafast transient absorption spectroscopy measurements, which show a faster bleach recovery for the PEI‐coated hybrid nanoparticles, consistent with faster and more efficient charge separation. These differences are primarily attributed to the effects of surface passivation by the different coatings affecting the surface trapping of charge carriers that compete with effective charge separation required for the photocatalysis. Further support of this assignment is provided from steady‐state emission and time‐resolved spectral measurements, performed on related strongly fluorescing CdSe/CdS nanorods. The control and understanding of the effect of the surface coating of the hybrid nanosystems on the photocatalytic processes is of importance for the potential application of hybrid nanoparticles as photocatalysts.     

Aqueous synthesis of CdSe and CdSe/CdS quantum dots with controllable introduction of Se and S sources

A new and convenient route is developed to synthesize CdSe and core–shell CdSe/CdS quantum dots (QDs) in aqueous solution. CdSe QDs are prepared by introducing H₂Se gas into the aqueous medium containing Cd²⁺ ions. The synthesized CdSe QDs are further capped with CdS to form core–shell CdSe/CdS QDs by reacting with H₂S gas. The gaseous precursors, H₂Se and H₂S, are generated on-line by reducing SeO₃ ^2? with NaBH₄ and the reaction between Na₂S and H₂SO₄, and introduced sequentially into the solution to form CdSe and CdSe/CdS QDs, respectively. The synthesized water-soluble CdSe and CdSe/CdS QDs possess high quantum yield (3 and 20 %) and narrow full-width-at-half-maximum (43 and 38 nm). The synthesis process is easily reproducible with simple apparatus and low-toxic chemicals. The relatively standard deviation of maxima fluorescence intensity is only 2.1 % (n = 7) for CdSe and 3.6 % (n = 7) for CdSe/CdS QDs. This developed route is simple, environmentally friendly and can be readily extended to the large-scale aqueous synthesis of QDs.     

Enhanced Photocatalytic H2‐Production Activity of CdS Quantum Dots Using Sn2+ as Cocatalyst under Visible Light Irradiation

Herein, oil‐soluble CdS quantum dots (QDs) are first prepared through a solvent‐thermal process. Then, oil‐soluble CdS QDs are changed into water‐soluble QDs via ligand exchange using mercaptopropionic acid as capping agent at pH 13. The photocatalytic performance is investigated under the visible light irradiation using glycerol as sacrificial agent and Sn²⁺ as cocatalyst. No H₂‐production activity is observed for oil‐soluble CdS QDs. Water‐soluble CdS QDs exhibit significantly enhanced hydrogen evolution rate. When the concentration of cocatalyst Sn²⁺ increases to 0.2 × 10^?3 m , the rate of hydrogen evolution reaches 1.61 mmol g^?1 h^?1, which is 24 times higher than that of the pristine water‐soluble CdS QDs. The enhanced H₂‐production efficiency is attributed to the adsorption of Sn²⁺ ions on the surface of CdS QDs that are further reduced to Sn atoms by photogenerated electrons. The in situ generated Sn atoms serve as photocatalytic cocatalyst for efficient hydrogen generation.     

Synthesis of cactus-like zincoxysulfide (ZnOxS1−x) nanostructures assisted by a task-specific ionic liquid and their photocatalytic activities

A hydrothermal method has been employed to prepare cactus-like zincoxysulfide ZnO_xS_1?x nanostructures with the assistance of a dicationic task-specific ionic liquid (TSIL), [mim]{(CH)₂}₃[imm](SCN)₂. To the best of our knowledge, this is the first time that this TSIL with the SCN anion has been used in place of conventional reagents as a source of S to prepare a ZnO_xS_1?x nanostructure. The photocatalytic activities of the ZnO_xS_1?x nanostructures have been compared using UV and visible lights. BET results showed that the surface areas and photocatalytic activities of cactus-like zincoxysulfide ZnO_xS_1?x nanostructures were higher than those of other samples. ZnO_xS_1?x nanostructures with cactus-like morphology exhibited a significant enhancement of photocatalytic activity toward the degradation of methyl orange (MO) as compared to other samples, as revealed by photoluminescence measurements. This could be attributed to enhanced oxygen vacancies and crystallite defects formed as a result of substitution of S^2? in the lattice of ZnO.     

Self‐Assembled Au/CdSe Nanocrystal Clusters for Plasmon‐Mediated Photocatalytic Hydrogen Evolution

Plasmon‐mediated photocatalytic systems generally suffer from poor efficiency due to weak absorption overlap and thus limited energy transfer between the plasmonic metal and the semiconductor. Herein, a near‐ideal plasmon‐mediated photocatalyst system is developed. Au/CdSe nanocrystal clusters (NCs) are successfully fabricated through a facile emulsion‐based self‐assembly approach, containing Au nanoparticles (NPs) of size 2.8, 4.6, 7.2, or 9.0 nm and CdSe quantum dots (QDs) of size ≈3.3 nm. Under visible‐light irradiation, the Au/CdSe NCs with 7.2 nm Au NPs afford very stable operation and a remarkable H₂‐evolution rate of (10× higher than bare CdSe NCs). Plasmon resonance energy transfer from the Au NPs to the CdSe QDs, which enhances charge‐carrier generation in the semiconductor and suppresses bulk recombination, is responsible for the outstanding photocatalytic performance. The approach used here to fabricate the Au/CdSe NCs is suitable for the construction of other plasmon‐mediated photocatalysts.     

Tuning photocurrent response through size control of CdSe quantum dots sensitized solar cells

The photovoltaic characterization of CdSe quantum dots sensitized solar cells (QDSSCs) by tuning band gap of CdSe quantum dots (QDs) through size control has been investigated. Fluorine doped tin oxide (FTO) substrates were coated with 20 nm in diameter TiO₂ nanoparticles (NPs). Pre-synthesized colloidal CdSe quantum dots of different sizes (from 4.0 to 5.4 nm) were deposited on the TiO₂-coated substrates using direct adsorption (DA) method. The FTO counter electrodes were coated with platinum, while the electrolyte containing I^?/I ₃ ^? redox species was sandwiched between the two electrodes. The current density-voltage (J-V) characteristic curves of the assembled QDSSCs were measured for different dipping times, and AM 1.5 simulated sunlight. The maximum values of short circuit current density (J_sc) and conversion efficiency (η) are 1.62 mA/cm² and 0.29 % respectively, corresponding to CdSe QDs of size 4.52 nm (542 nm absorption edge) and of 6 h dipping time. The variation of the CdSe QDs size mainly tunes the alignment of the conduction band minimum of CdSe with respect to that of TiO₂ surface. Furthermore, the J_sc increases linearly with increasing intensity of the sun light, which indicates the sensitivity of the assembled cells.     

Photocatalytic H2 evolution over sulfur vacancy-rich ZnIn2S4 hierarchical microspheres under visible light

In this work, the sulfur vacancies were successfully introduced into the ZnIn₂S₄ (ZIS) lattice through two facile approaches, plasma etching and annealing, for enhancing the photocatalytic performance. The optimized plasma-etched ZIS exhibited an enhanced H₂ generation rate of 706 μmol g^?1 h^?1, which was 5 and 1.2 times higher than that of pure ZIS and annealed ZIS, respectively. Theoretical calculation demonstrated that surface S vacancy could arouse the catalytic activity of the adjacent S atoms in inert (001) basal plane, serving as the active site for hydrogen evolution reaction (HER). Although annealing could produce much more S vacancies than the plasma etching, a majority of bulk S vacancies usually acted as charge recombination center to lower the photocatalytic activity. Hence, even plasma-etched ZIS presented poor light absorption capacity, plasma etching showed a better effect on the HER improvement of ZIS than annealing. This work presents a simple and promising pathway for optimization of 3D ZIS photocatalysts to improve photocatalytic hydrogen evolution.

     

Dibenzothiophene‐S,S‐Dioxide‐Based Conjugated Polymers: Highly Efficient Photocatalyts for Hydrogen Production from Water under Visible Light

Three dibenzothiophene‐S,S‐dioxide‐based alternating copolymers were synthesized by facile Suzuki polymerization for visible light–responsive hydrogen production from water (> 420 nm). Without addition of any cocatalyst, FluPh2‐SO showed a photocatalytic efficiency of 3.48 mmol h^?1 g?¹, while a larger hydrogen evolution rate (HER) of 4.74 mmol h^?1 g^?1 was achieved for Py‐SO, which was ascribed to the improved coplanarity of the polymer that facilitated both intermolecular packing and charge transport. To minimize the possible steric hindrance of FluPh2‐SO by replacing 9,9′‐diphenylfluorene with fluorene, Flu‐SO exhibited a more red‐shifted absorption than FluPh2‐SO and yielded the highest HER of 5.04 mmol h^?1 g^?1. This work highlights the potential of dibenzothiophene‐S,S‐dioxide as a versatile building block and the rational design strategy for achieving high photocatalytic efficiency.     

Hole transport in organic field-effect transistors with active poly(3-hexylthiophene) layer containing CdSe quantum dots

Hybrid field-effect transistors (FETs) based on poly(3-hexylthiophene) (P3HT) containing CdSe quantum dots (QDs) were fabricated. The effect of the concentration of QDs on charge transport in the hybrid material was studied. The influence of the QDs capping ligand on charge transport parameters was investigated by replacing the conventional trioctylphosphine oxide (TOPO) surfactant with pyridine to provide closer contact between the organic and inorganic components. Electrical parameters of FETs with an active layer made of P3HT:CdSe QDs blend were determined, showing field-effect hole mobilities up to 1.1×10^?4 cm²/Vs. Incorporation of TOPO covered CdSe QDs decreased the charge carrier mobility while the pyridine covered CdSe QDs did not alter this transport parameter significantly.     

Surface modification of CdSe quantum dots for biosensing applications: Role of ligands

For an optimum performance of colloidal nanocrystal devices for a variety of applications such as photonic devices, solar cells and biological labelling, the determining factors are the nanocrystal surface and size. In this work, these two factors have been tuned via wet chemistry to tailor the material properties: The absorption and emission spectra have been tailored by choice of the nanocrystal size; nanocrystal shape by surface modification and photoluminescence (PL) efficiency determined by surface traps, has been tuned via appropriate selection of the nanocrystal capping ligands. Here, we have shown that through ligand-exchange process, the surface of the CdSe quantum dots (QDs) can be modified by replacing the longer-chain ligands of conventional trioctyl phosphine oxide (TOPO) or oleic acid (OA) with shorter-chain ligand of butyl amine. This imparts colloidal stability and water solubility to CdSe QDs for its potential applications in biosensors and biological imaging. It is conjectured that crystallite sizes, oxidation potential of CdSe QDs and stereochemical compatibility of ligands (TOPO, oleic acid and butyl amine) greatly influences the photophysics and photochemistry of CdSe QDs.     

Enhanced electron transfer rate for quantum dot sensitized solar cell based on CNT-TiO2 film

In this paper, we have fabricated a quantum dot sensitized solar cell (QDSSC) based on carbon nanotube (CNT) doped TiO2 mesopores film. As revealed by field emission scanning electron microscopy and absorption spectra, the CdSe QDs were adsorbed onto CNT-TiO2 nanocomposite. An improved efficiency is achieved for the CNT-TiO2/CdSe devices compared to that of TiO2/CdSe, which is due to the increased surface area and reduced charge recombination in TiO2 film by the presence of CNTs. A power conversion efficiency of the as-prepared QDSSC of 0.98% was obtained under 100 mW/cm2 solar irradiation. The emission decay profile demonstrates that the electrons transfer from CdSe QDs to CNT-TiO2 is faster than that from CdSe QDs to TiO2, resulting in the reduction of the charge recombination, leading to a higher FF value in QDSSC. The average lifetime of CdSe QDs adsorbed on TiO2 doped with CNT is 6.2 ns and the electron transfer rate constant of 1.1 x 10(8) s(-1) can be calculated.     

Highly Selective Photoreduction of CO2 with Suppressing H2 Evolution by Plasmonic Au/CdSe–Cu2O Hierarchical Nanostructures under Visible Light

Here, the photocatalytic CO₂ reduction reaction (CO₂RR) with the selectivity of carbon products up to 100% is realized by completely suppressing the H₂ evolution reaction under visible light (λ > 420 nm) irradiation. To target this, plasmonic Au/CdSe dumbbell nanorods enhance light harvesting and produce a plasmon‐enhanced charge‐rich environment; peripheral Cu₂O provides rich active sites for CO₂ reduction and suppresses the hydrogen generation to improve the selectivity of carbon products. The middle CdSe serves as a bridge to transfer the photocharges. Based on synthesizing these Au/CdSe–Cu₂O hierarchical nanostructures (HNSs), efficient photoinduced electron/hole (e^?/h⁺) separation and 100% of CO selectivity can be realized. Also, the 2e^?/2H⁺ products of CO can be further enhanced and hydrogenated to effectively complete 8e^?/8H⁺ reduction of CO₂ to methane (CH₄), where a sufficient CO concentration and the proton provided by H₂O reduction are indispensable. Under the optimum condition, the Au/CdSe–Cu₂O HNSs display high photocatalytic activity and stability, where the stable gas generation rates are 254 and 123 µmol g^?1 h^?1 for CO and CH₄ over a 60 h period.     

Investigation of the Photocatalytic Hydrogen Production of Semiconductor Nanocrystal-Based Hydrogels

Destabilization of a ligand-stabilized semiconductor nanocrystal solution with an oxidizing agent can lead to a macroscopic highly porous self-supporting nanocrystal network entitled hydrogel, with good accessibility to the surface. The previously reported charge carrier delocalization beyond a single nanocrystal building block in such gels can extend the charge carrier mobility and make a photocatalytic reaction more probable. The synthesis of ligand-stabilized nanocrystals with specific physicochemical properties is possible, thanks to the advances in colloid chemistry made in the last decades. Combining the properties of these nanocrystals with the advantages of nanocrystal-based hydrogels will lead to novel materials with optimized photocatalytic properties. This work demonstrates that CdSe quantum dots, CdS nanorods, and CdSe/CdS dot-in-rod-shaped nanorods as nanocrystal-based hydrogels can exhibit a much higher hydrogen production rate compared to their ligand-stabilized nanocrystal solutions. The gel synthesis through controlled destabilization by ligand oxidation preserves the high surface-to-volume ratio, ensures the accessible surface area even in hole-trapping solutions and facilitates photocatalytic hydrogen production without a co-catalyst. Especially with such self-supporting networks of nanocrystals, the problem of colloidal (in)stability in photocatalysis is circumvented. X-ray photoelectron spectroscopy and photoelectrochemical measurements reveal the advantageous properties of the 3D networks for application in photocatalytic hydrogen production.     

High Edge Selectivity of In Situ Electrochemical Pt Deposition on Edge‐Rich Layered WS2 Nanosheets

Recent studies show that the Pt electrode can be slowly dissolved into the acidic media and regenerate on the working electrode along with the long‐time hydrogen evolution reaction (HER) test. However, to date, the relationship between the Pt deposition and the intrinsic properties of the working electrode remains elusive. Herein, for the first time, the edge selectivity of in situ electrochemical Pt deposition on layered 2H‐WS₂ nanosheets, whose edge surface with rich dangling bonds is chemically active to regulate their properties, especially the interfacial reaction occurring between the electrode surface and the adjacent thin layer of the solution, is theoretically elucidated and experimentally verified by controlling the cathode polarization test using Pt wire as the counter electrode in H₂SO₄ solution. It is revealed that the layered WS₂ nanosheets with rich exposed edges show much stronger interaction with Pt atoms because the terminated S₂^2? or S^2? ligands on the edge exhibit much lower binding energy for Pt atoms compared with the apical S^2? ligands on the terrace surface. The in situ electrochemical Pt‐deposited WS₂ nanosheets with rich exposed edges can act as a highly active hybrid electrocatalyst to accelerate HER kinetics and exhibit commercial Pt‐like HER performance, especially in the alkaline media.     

Giant Zeeman Effects and Spin Dynamics of Excitons in Dense Self-Organized Quantum Dots of CdSe and Cd1?xMnxSe

Giant Zeeman effects and spin dynamics of excitons are studied in dense self-organized quantum dots (QDs) of CdSe and Cd_1–xMn_xSe. Microphotoluminescence (PL) measurements for each individual dot reveal the typical dot diameter of 3.5 ± 0.2 nm and the density of 5000 m^–2 in the CdSe QDs. The exciton lifetime is shorter in smaller dots with higher energies, indicating energy transfer and tunneling processes among the dots. Circular polarization of excitonic PL is observed at 0 T with an opposite sign to that of the excited light and with the rise time of 50 ps. The CdSe QDs coupled with a Zn_1–xMn_xSe layer show the giant Zeeman shift of exciton, arising from overlapping of exciton wavefunctions in the dots with Mn ions. Spin polarization dynamics in the coupled QDs is also studied.     

Giant Zeeman Effects and Spin Dynamics of Excitons in Dense Self-Organized Quantum Dots of CdSe and Cd<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Mn<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Se

Giant Zeeman effects and spin dynamics of excitons are studied in dense self-organized quantum dots (QDs) of CdSe and Cd_1–xMn_xSe. Microphotoluminescence (PL) measurements for each individual dot reveal the typical dot diameter of 3.5 ± 0.2 nm and the density of 5000 m^–2 in the CdSe QDs. The exciton lifetime is shorter in smaller dots with higher energies, indicating energy transfer and tunneling processes among the dots. Circular polarization of excitonic PL is observed at 0 T with an opposite sign to that of the excited light and with the rise time of 50 ps. The CdSe QDs coupled with a Zn_1–xMn_xSe layer show the giant Zeeman shift of exciton, arising from overlapping of exciton wavefunctions in the dots with Mn ions. Spin polarization dynamics in the coupled QDs is also studied.     

Phase transfer of hydrophobic QDs for water-soluble and biocompatible nature through silanization

A novel method has been developed for creating water-soluble and biocompatible CdSe/ZnS quantum dots (QDs) with a small hydrodynamic diameter (less than 10 nm). The silanization of the QDs was carried out by using partially hydrolyzed tetraethyl orthosilicate (TEOS) to replace organic ammine or tri-n-octylphosphine oxide on the surface of the QDs. The partially hydrolyzed 3-mercaptopropyltrimethoxysilane attached to the hydrolyzed TEOS layer on the QDs prevented the QDs from agglomeration when the QDs were transferred into water. The functional SiO₂-coated QDs were conjugated with immunoglobin G antibody by using biotin-streptavidin as linkers. The SiO₂-coated QDs exhibited the same absorption and photoluminescence (PL) spectra as those of initial QDs in organic solvents. The SiO₂-coated QDs preserved PL intensities, is colloidally stable over a wide pH range (pH 6-11). Because the mean diameter of amphiphilic polymer-coated QDs was almost 2 times of that of functional SiO₂-coated QDs, the QD phase transfer by silanization is a well-established method for generating biocompatible QDs.     

Synthesis and size dependent optical studies in CdSe quantum dots via inverse micelle technique

Cadmium selenide quantum dots (CdSe QDs) were successfully synthesized without using trioctylphosphine (TOP). The XRD pattern showed zinc-blend phase of the CdSe QDs. The absorption and PL spectra exhibit a strong blue shift as the QDs size decreases due to the quantum confinement effect. In addition, the quantum efficiency of CdSe QDs with TOP capping is higher than CdSe QDs with oleic acid capping. TEM image shows a spherical shape, compact and dense structure of CdSe QDs. A good agreement between the Tauc's model and experimentally measured absorption spectra of CdSe QDs is achieved. The FTIR peak at ～1712 cm^?1 spectra confirms the influence of oleic acid as a capping agent.     

Facile synthesis of Bi2S3 nanocrystalline-modified TiO2: Fe nanotubes hybrids and their photocatalytic activities in dye degradation

Interface-induced effects and large specific area of heteronanostructures are attracting much attention due to applications in photocatalysis. In this work, ultrafine bismuth sulfide (Bi₂S₃) nanocrystalline-modified Fe-doped TiO₂ nanotubes (NTs) were fabricated with facile methods. The effect of the ratio of Bi₂S₃ to the Fe-doped TiO₂ NTs on the microstructural, optical, and photocatalytic properties of the NTs and hybrids was studied. The NTs showed an actual Fe content of ～ 2.93 at.%. The optical bandgap of the NTs and hybrids was ～2.90 eV and ～2.46–2.88 eV, respectively, and decreased with increasing Bi₂S₃/NTs ratio. The specific surface area of the NTs was ～333 m² · g^?1; whereas the hybrids showed obviously larger specific surface area of ～ 527–689 m² · g^?1 than the NTs because of well-controlled formation process of Bi₂S₃ nanoparticles. The sunlight-excited degradation experiments of dyes in the water indicated that the photocatalytic activity of the hybrids was higher than that of the NTs and increased with increasing Bi₂S₃/NTs ratio. Moreover, the degradation rates of two dyes at different initial pH values were very different. The high photocatalytic activity of the hybrids was mainly ascribed to the narrow bandgap, large specific surface area, and effective heterojunction.