CdSe heterostructure catalysis

Mixed-dimensional heterostructures (MDHs), which combine nanomaterials of different dimensionalities deliver on the promise to bypass intrinsic limitations of a given low-dimensional material. Here, a strategy to engineer MDHs between two low-dimensional materials by curvature-complementary self-assembly is described. CdSe nanotubes rolled from 2D nanosheets and 1D CdSe nanorods, with negative and positive curvatures, respectively, are selected to illustrate complementary curvature self-assembly. The assembly process, optical, and photoelectrical properties of the CdSe MDHs are thoroughly investigated. Several remarkable features of CdSe MDHs, including increased light absorption, efficient charge separation, and appropriate bandgap structure are confirmed. The MDHs significantly alleviate the sluggish kinetics of electron transfer in the quantum sized CdSe subunits (onset potential of 0.21 V vs RHE for MDHs; 0.4 V lower than their low-dimensional building blocks), while the spatial nano-confinement effect in the CdSe MDHs also assists the interfacial reaction kinetics to render them ideal photocatalysts for benzylamine oxidation (conversion > 99% in 4 h with a two times higher rate than simple mixtures). The results highlight opportunities for building MDHs from low-dimensional building blocks with curvature-complementary features and expand the application spectrum of low dimensional materials in artificial photosynthesis.

The design of nano-heterostructures for light-harvesting systems for photocatalysis and photovoltaic applications is an emerging area of research. Here, we report the synthesis of a one-dimensional quasi-type-II CdS/CdSe heterostructure where holes are confined in CdSe nanoparticles and electrons can delocalize throughout the conduction bands of both CdS nanorods and CdSe nanoparticles because of the smaller conduction band offset. By controlling the oxidation and reduction sites of the CdS/CdSe heterostructure, we achieved a maximum H2 generation of 5125 μmol/g/h for 27.5 wt % CdSe-loaded CdS heterostructure, which is found to be 44 times higher than that of bare CdS nanorods and 22 times higher than that of CdSe nanoparticles. Furthermore, this heterostructure exhibits a photovoltaic effect (Voc = 0.8 V, Jsc = 0.56 mA/cm2, FF = 40%, and η = 0.18), which could be useful for solar cell application. The bleaching recovery kinetics and hot electron cooling dynamics have been studied by using femtosecond transient spectroscopy, which confirms the efficient charge separation and long excited-state lifetime of 27.5% CdSe-loaded CdS heterostructure. Thus, the slow recombination process is the reason for efficient H2 generation and photovoltaic properties.

A facile synthetic route for TiO2-CdSe heterostructures was proposed based on dentate binding of TiO2 to carboxyl. Carboxyl functionalized CdSe quantum dots (CF-CdSe QDs) were successfully bonded onto TiO2 nanoparticles (NPs), which could significantly improve the photoelectrochemical (PEC) properties of TiO2 NPs. This is ascribed to the fact that CdSe QDs with a narrow band gap could be stimulated under visible light irradiation, and the energy levels of TiO2 NPs and CF-CdSe QDs are aligned with an electrolyte solution. High resolution transmission electron microscopy images revealed the heterostructures of the TiO2-CdSe composites. Ultraviolet visible spectroscopy, photoluminescence emission spectroscopy and electrochemical impedance spectroscopy analysis exhibited that the prepared TiO2-CdSe heterostructures have improved light absorption, charge separation efficiency and electron transfer ability in the visible light region. TiO2-CdSe heterostructures were used as versatile labels for fabrication of PEC and electrochemical immunosensors, and human immune globulin G (HIgG) was used as a model analyte. The immunosensor showed high sensitivity, a low detection limit and a wide linear range, which could be applied in practical serum sample analysis. The constructed TiO2-CdSe heterostructures would have potential applications in photocatalysis, aptasensors, cytosensors and other areas of nanotechnology.

Semiconductor-metal hybrid nanostructures are recognized as great materials due to their high level of light-induced charge separation, which has direct relevance in photocatalysis and solar energy conversion. To understand the mechanism of charge separation processes, hybrid CdSe@CdS{Au} nano-heterostructures containing Au nanoparticles (NPs) with different sizes were synthesized, and the ultrafast charge-transfer dynamics were monitored using femtosecond transient absorption spectroscopy. Steady-state optical absorption studies suggest the formation of charge-transfer complexes between core shell nanocrystals (NCs) and Au NPs. Steady-state and time-resolved luminescence spectroscopy suggest electron transfer from the photo-excited CdSe@CdS core shell QDs NCs to the Au NPs within the heterostructure. The ultrafast interfacial electron-transfer dynamics in the heterostructures were monitored by femtosecond transient absorption spectroscopy. The results revealed that both hot and thermalized electrons are transferred from the core shell QDs to the metal NPs with time constants of 150 and 300 fs, respectively. Hot-electron transfer from QDs to Au NPs was found to take place predominantly in the heterostructures depending on the sizes of the metal NPs. The photo-degradation of rhodamin B in the presence of the CdSe@CdS{Au} heterostructures under visible-light radiation suggests that the hot electrons in the heterostructures play a major role in photocatalytic degradation.

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The metal-semiconductor heterostructure is an important candidate for photocatalysis due to its efficient charge transport and separation. A controllable morphology and ideal interfaces are critically significant for improving the heterostructure photocatalytic performance. By controlling the concentration of Cd²⁺ to control the reaction environment (pH value) and reaction rate, the CdSe nanocrystal is overgrown on the side or tip of the Au nanorods, which leads to a strong interaction between the excitons of CdSe nanocrystals and the plasmons of Au nanorods. Both kinds of Au-CdSe heterorods exhibit good hydrogen productivity. Particularly, the lateral Au-CdSe heterorods exhibit excellent photocatalytic efficiency due to the larger contact interface of Au and CdSe and the strong local field of the CdSe nanocrystals grown on one side of the Au nanorods being enhanced by the transverse plasmon resonance in the visible region. We provide an approach to modulate the combination of the asymmetric metal nanoparticle and the semiconductor shell; these core-semishell heterostructures have potential applications ranging from photocatalysis to photonic nanodevices.

Fabricating efficient photocatalysts with rapid charge carrier separation and high visible light harvesting is an advisable strategy to improve CO₂ reduction performance. Herein, hierarchical Co_0.85 Se-CdSe/MoSe₂ /CdSe cages with sandwich-like heterostructure are prepared to act as efficient photocatalysts for CO₂ reduction. In this study, the structure and composition of the final products can be regulated through the cation-exchange reaction in the presence of ascorbic acid. In the Co_0.85 Se-CdSe/MoSe₂ /CdSe cages, MoSe₂ nanosheets function as a bridge to integrate Co_0.85 Se-CdSe and CdSe on both sides of the MoSe₂ nanosheet shell into a sandwich-like heterostructured catalyst system, which possesses multiple positive merits for photocatalysis, including accelerated transport and separation of photogenerated carriers, improved visible light utilization, and increased catalytic active sites. Thus, the optimized Co_0.85 Se-CdSe/MoSe₂ /CdSe cages exhibit remarkable visible-light photocatalytic performance and outstanding stability for CO₂ reduction with a high CO average yield of 15.04 µmol g^-1 h^-1 and 90.14% selectivity, which are much higher than those of other control samples including single-component catalysts and binary hybrid catalysts. This study provides a promising way for the design and fabrication of high-efficiency photocatalysts.

Tackling the complex challenge of harvesting solar energy to generate energy-dense fuels such as hydrogen requires the design of photocatalytic nanoarchitectures interfacing components that synergistically mediate a closely interlinked sequence of light-harvesting, charge separation, charge/mass transport, and catalytic processes. The design of such architectures requires careful consideration of both thermodynamic offsets and interfacial charge-transfer kinetics to ensure long-lived charge carriers that can be delivered at low overpotentials to the appropriate catalytic sites while mitigating parasitic reactions such as photocorrosion. Here we detail the theory-guided design and synthesis of nanowire/quantum dot heterostructures with interfacial electronic structure specifically tailored to promote light-induced charge separation and photocatalytic proton reduction. Topochemical synthesis yields a metastable β-Sn_0.23V₂O₅ compound exhibiting Sn 5s-derived midgap states ideally positioned to extract photogenerated holes from interfaced CdSe quantum dots. The existence of these midgap states near the upper edge of the valence band (VB) has been confirmed, and β-Sn_0.23V₂O₅/CdSe heterostructures have been shown to exhibit a 0 eV midgap state-VB offset, which underpins ultrafast subpicosecond hole transfer. The β-Sn_0.23V₂O₅/CdSe heterostructures are further shown to be viable photocatalytic architectures capable of efficacious hydrogen evolution. The results of this study underscore the criticality of precisely tailoring the electronic structure of semiconductor components to effect rapid charge separation necessary for photocatalysis.

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Semiconductor quantum dots (QDs) have garnered tremendous attention by virtue of their substantial light-harvesting and conversion efficiencies, large number of active sites, unique quantum size confinement, and multiple exciton generation effects. In this regard, recent years have witnessed their widespread applications in photocatalysis. Nonetheless, intrinsic disadvantages of QDs including unfavorable photostability, ultrafast charge recombination rate, and sluggish kinetic of charge carriers retard the construction of high-efficiency QDs-based photocatalysts for solar energy conversion. Thus far, in-depth investigation on the visible-light-driven photoredox organic transformation over QDs has not yet been exhaustively explored, and corresponding photocatalytic mechanisms remain elusive. In this work, selecting cadmium selenide (CdSe) as a quintessential category of semiconductor QDs, we demonstrated a facile, green, easily accessible, and rather efficient electrostatic self-assembly strategy to conspicuously boost the versatile photoredox performances of CdSe QDs toward selective organic transformation under visible light irradiation by intimately integrating with graphene (GR) via judicious surface charge tuning. In this scenario, intrinsically negatively charged CdSe QDs and surface-modified positively charged GRs were utilized as the building blocks for spontaneous electrostatic self-assembly buildup, which gives rise to well-defined CdSe QDs–GR ensembles. More intriguingly, ligands capped on the CdSe QDs surface enable the alternate layer-by-layer (LbL) assembly of CdSe QDs and GR forming three-dimensional spatially multilayered heterostructures. Furthermore, it was significant to unveil that such self-assembled CdSe QDs–GR nanocomposites exhibit remarkably enhanced and multifunctional photoredox performances toward selective oxidation of aromatic alcohols to corresponding aromatic aldehydes and selective reduction of nitroaromatics to amino compounds under visible light irradiation, which far exceeds the pristine CdSe QDs counterpart, which exhibits almost negligible photoactivities. This can be ascribed to the pivotal role of GR for conspicuously capturing and shuttling electrons from band-gap photoexcitation of CdSe QDs, intimate interfacial contact between the building blocks, and enlarged specific surface area stemming from seamless GR encapsulation and intercalation, along with the unique ligand-triggered LbL assembly integration mode between CdSe QDs and GR, hence synergistically reducing the recombination rate and prolonging the lifetime of charge carriers. Furthermore, photoredox mechanisms of the CdSe QDs–GR ensemble were elucidated. It is anticipated that our work would afford an efficacious avenue to finely modulate the charge transport over QDs for solar energy conversion.

This work presents a hitherto unreported approach to assemble a 1D oxide-1D chalcogenide heterostructured photoactive film. As a representative system, bismuth (Bi) catalyzed 1D CdSe nanowires are directly grown on anodized 1D TiO₂ nanotube (T_NT). A combination of the reductive successive-ionic-layer-adsorption-reaction (R-SILAR) and the solution-liquid-solid (S-L-S) approach is implemented to fabricate this heterostructured assembly, reported in this 1D/1D form for the first time. XRD, SEM, HRTEM, and elemental mapping are performed to systematically characterize the deposition of bismuth on T_NT and the growth of CdSe nanowires leading to the evolution of the 1D/1D heterostructure. The resulting "treelike" photoactive architecture demonstrates UV-visible light-driven electron-hole pair generation. The photoelectrochemical results highlight: (i) the formation of a stable n-n heterojunction between TiO₂ nanotube and CdSe nanowire, (ii) an excellent correlation between the absorbance vis-à-vis light conversion efficiency (IPCE), and (iii) a photocurrent density of 3.84 mA/cm². This proof-of-concept features the viability of the approach for designing such complex 1D/1D oxide-chalcogenide heterostructures that can be of interest to photovoltaics, photocatalysis, environmental remediation, and sensing.

Abstract A synthesis and characterization of luminescent nano‐heterostructures consisting of CdSe nanorod (NR) cores and a ZnO shell with up to three monolayers of ZnO is reported. The core/shell heterostructures show a tunable, dual photoluminescence (PL) in visible and Near Infrared (NIR) spectral ranges. Upon shelling the visible PL band attributed to the carrier recombination within the CdSe core shifts to lower energy by ≈0.05 to 0.15 eV relative to the bare CdSe NRs, due to a reduced quantum confinement. A NIR band, observed ≈0.4 – 0.5 eV below the PL energy of the CdSe core, is attributed to a type‐II carrier recombination across the CdSe/ZnO interface. The total PL quantum yield (PLQY) in the brightest heterostructures reaches ≈20%, increasing ≈100‐fold over the PLQY of the corresponding bare CdSe NRs. The average lifetimes of the visible PL in some heterostructures exceeds 100 ns, compared to ≈5 ns lifetime typical for bare CdSe NRs. The average PL lifetimes attributed to the type‐II charge separated states exceed one microsecond. Strong NIR PL, tunable in the 800–900 nm spectral range and the long‐lived charge separated state make the CdSe/ZnO core‐shell NRs appealing materials for exploitation in applications such as bioimaging, photocatalysis and optoelectronics.

Semiconductor-catalyst heterostructures have shown promising performances for light-driven H₂ generation, although further development of these materials is hindered by the lack of cost-effective and efficient catalysts. In this paper, we adopt a colloidal method to prepare few-layer WSe₂ nanosheets without exfoliation and apply them as catalysts for forming heterostructures with a wide range of semiconductor absorbers (CdS nanorods, CdSe/CdS dot-in-rods, TiO₂ nanoparticles, g-C₃N₄ nanosheets). These WSe₂-semiconductor heterostructures show enhanced solar-to-hydrogen conversion efficiencies compared to semiconductors without WSe₂. The detailed mechanism of this enhancement has been investigated using WSe₂ nanosheet-decorated CdSe/CdS dot-in-rods as a model system, which display ∼5.5-fold higher hydrogen generation apparent quantum efficiency compared to free CdSe/CdS dot-in-rods. Transient absorption spectroscopic studies reveal efficient charge separation in WSe₂-decorated CdSe/CdS dot-in-rods, suggesting its key role in enhancing the H₂ generation efficiency of WSe₂-semiconductor heterostructures. This work demonstrates the great potentials of WSe₂ nanosheets as catalysts for light-driven hydrogen production and the important effect of forming WSe₂-semiconductor heterostructures in facilitating charge separation and photocatalysis.

Zn chalcogenides are suitable candidates for blue-emitting fluorophores in light-emitting devices. In particular, the efforts to grow ZnSe nanocrystals (NCs) with fine control over size and shape via bottom-up approaches have faced challenges because of the slow decomposition of Zn precursors. In this study, we report direct cation exchange from CdSe NCs to ZnSe. Absorption spectroscopy and density functional theory (DFT) analysis reveal that the reactivity of cation exchange depends on the degree of complexation between organic ligands and Zn halides. We controlled the binding strength of Zn complexes by changing the organic ligands and halogen species that bind with Zn. Appropriate binding strength allows for the release of Zn ions and their facile incorporation into CdSe seed NCs. Under our experimental conditions, trioctylphosphine oxide (TOPO)-ZnI2 drives the efficient cation exchange reaction whereas TOPO-ZnCl2 induces no cation exchange of CdSe NCs. In addition, functional groups vary the binding strength between Zn and ligands. Oleylamine (OLAm)-ZnI2, which has a weaker ligand-ZnI2 binding than TOPO-ZnI2, breaks down the original morphologies of host CdSe NCs due to the very fast exchange rate. On the other hand, the TOPO-ZnI2 complex induces a mild exchange rate, leading to transformation into various morphologies such as CdSe nanorods (NRs) and nanoplatelets (NPLs) into CdSe/ZnSe heterostructures inaccessible via other synthesis methods. The incorporation of Zn into various morphologies of CdSe results in tunable optical transitions in blue-UV regions. The synthesis of heterostructured NCs in an elongated morphology is possible, opening opportunities in photocatalysis, light emitting diodes, and luminescent solar concentrators.

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We synthesize colloidal CdSe@CdS octapod nanocrystals decorated with Pt domains, resulting in a metal-semiconductor heterostructure. We devise a protocol to control the growth of Pt on the CdS surface, realizing both a selective tipping and a non-selective coverage. Ultrafast optical spectroscopy, particularly femtosecond transient absorption, is employed to correlate the dynamics of optical excitations with the nanocrystal morphology. We find two regimes for capture of photoexcited electrons by Pt domains: a slow capture after energy relaxation in the semiconductor, occurring in tipped nanocrystals and resulting in large spatial separation of charges, and an ultrafast capture of hot electrons occurring in nanocrystals covered in Pt, where charge separation happens faster than energy relaxation and Auger recombination. Besides the relevance for fundamental materials science and control at the nanoscale, our nanocrystals may be employed in solar photocatalysis.

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Semiconductor nanocrystals of tunable shell/core configurations have great potential in photo-driven applications such as photoluminescence and photocatalysis, but few strategies realize a controllable synthesis with respect to both the size of the core and the shell with high crystallinity. Here, a new synthetic method based on cadmium cyanamide (CdNCN) nanoparticle anion exchange reactions was developed to access solid or hollow CdSe nanocrystals with tunable size and CdNCN@CdS heterostructures with modulated shell/core thickness. The gradual shift and narrow width of photoluminescence features demonstrate the high crystallinity and monodispersity of the resulting CdSe nanocrystals. In the CdNCN@CdS heterostructures, synergistic effects of the photocarrier separation is observed between the CdS shell and CdNCN core, which leads to great improvement in photocatalysis with optimized shell/core ratio.

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We synthesized a new class of heterostructures by depositing CdS, CdSe, or CdTe quantum dots (QDs) onto α-V₂O₅ nanowires (NWs) via either successive ionic layer adsorption and reaction (SILAR) or linker-assisted assembly (LAA). SILAR yielded the highest loadings of QDs per NW, whereas LAA enabled better control over the size and properties of QDs. Soft and hard x-ray photoelectron spectroscopy in conjunction with density functional theory calculations revealed that all α-V₂O₅/QD heterostructures exhibited Type-II band offset energetics, with a staggered gap where the conduction- and valence-band edges of α-V₂O₅ NWs lie at lower energies (relative to the vacuum level) than their QD counterparts. Transient absorption spectroscopy measurements revealed that the Type-II energetic offsets promoted the ultrafast (10^-12-10^-11 s) separation of photogenerated electrons and holes across the NW/QD interface to yield long-lived (10^-6 s) charge-separated states. Charge-transfer dynamics and charge-recombination time scales varied subtly with the composition of heterostructures and the nature of the NW/QD interface, with both charge separation and recombination occurring more rapidly within SILAR-derived heterostructures. LAA-derived α-V₂O₅/CdSe heterostructures promoted the photocatalytic reduction of aqueous protons to H₂ with a 20-fold or greater enhancement relative to isolated colloidal CdSe QDs or dispersed α-V₂O₅ NWs. The separation of photoexcited electrons and holes across the NW/QD interface could thus be exploited in redox photocatalysis. In light of their programmable compositions and properties and their Type-II energetics that drive ultrafast charge separation, the α-V₂O₅/QD heterostructures are a promising new class of photocatalyst architectures ripe for continued exploration.

Carbodiimide-mediated coupling chemistry was used to synthesize heterostructures of CdSe and CdTe quantum dots (QDs) with varying ratios of electron-donating CdTe QDs and electron-accepting CdSe QDs. Heterostructures were assembled via the formation of amide bonds between the terminal functional groups of CdTe-adsorbed 4-aminothiophenol (4-ATP) ligands and CdSe-adsorbed N-hydroxysuccinimide (NHS) ligands. The number of charge acceptors on the surfaces of QDs can greatly influence the rate constant of excited-state charge transfer with QDs capable of accommodating far more acceptors than molecular chromophores. We report here on excited-state electron transfer within heterostructure-forming mixtures of 4-ATP-capped CdTe and NHS-capped CdSe QDs with varying molar ratios of CdTe to CdSe. Photophysical properties and charge transfer were characterized using UV-vis absorption, steady-state emission, and time-resolved emission spectroscopy. As the relative concentration of electron-accepting CdSe QDs within mixtures of 4-ATP-capped CdTe and NHS-capped CdSe QDs increased, the rate and efficiency of electron transfer increased by 100-fold and 7.4-fold, respectively, as evidenced by dynamic quenching of band-edge emission from CdTe QDs. In contrast, for non-interacting mixtures of thiophenol capped CdTe QDs and NHS-capped CdSe QDs, which served as control samples, photophysical properties of the constituent QDs were unperturbed and excited-state charge transfer between the QDs was negligible. Our results reveal that carbodiimide-mediated coupling chemistry can be used to control the relative number of donor and acceptor QDs within heterostructures, which, in turn, enables fine-tuning of charge-transfer dynamics and yields. These amide-bridged dual-QD heterostructures are, thus, intriguing for light harvesting, charge transfer, and photocatalysis.

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Abstract The quality of heterojunctions at the quantum dot (QD)‐TiO 2 nanotube (TNT) interface has important implications on the efficiencies of photoelectrochemical solar cells. Here, it is shown that electrophoretic deposition of pre‐synthesized thioacid‐capped CdTe QDs results in relatively poor charge transfer across the heterojunctions. This is likely due to the intermediate layer of bifunctional linkers (S‐R‐COOH) in between the QDs and TNT. On the other hand, CdTe QD‐sensitized TNT prepared by in situ deposition in aqueous medium provides direct QD‐TNT contact, and hence more favorable heterojunction for charge transfer. This is exemplified not only by the drastic improvement in photocurrent efficiencies, but also provides clear difference on the size‐dependent electron injection efficiencies from the CdTe QDs of different sizes. By extending the system further to CdSe QDs, drastic enhancement is found when carrying out the in situ deposition in an organic medium. The results are discussed in terms of the nature of deposition and the corresponding charge transport characteristics. More importantly, the work reflects the intricacy of the effects of QD size and the quality of the heterojunctions on the overall photoconversion efficiencies.

Presence of heterojunctions is important for generation of free charge carriers and the dissociation of bound electron-hole pairs in semiconductor nanoparticles. This work presents a theoretical investigation of the effect of core/shell heterojunction on electron-hole interaction in CdSe/ZnS quantum dots. The excitonic wave function in the CdSe/ZnS dots was calculated using the electron-hole explicitly correlated Hartree-Fock (eh-XCHF) method and the effect of successive addition of the ZnS shell on exciton binding energy, electron-hole recombination probability, and the electron-hole separation distance was investigated. It was found that the scaling of all the three quantities as a function of dot diameter did not follow conventional volume scaling laws of core-only dots, and the scaling laws were significantly altered due to the presence of the heterojunction. The spatial localization of the quasiparticles in the core/shell quantum dot was analyzed by calculating the 1-particle reduced density from the eh-XCHF wave function and partitioning the density spatially into core and shell regions. It was found that in the 15 nm CdSe/ZnS dot, the relative probability of the electron localization in the shell region was higher than the hole by a factor of 3. The degree of spatial localization of the quasiparticles was found to depend strongly on the initial size of the CdSe core in the core/shell quantum dot. It was found that a reduction in the CdSe core diameter by a factor of 1.7 resulted in an enhancement of the preferential localization of the electron in the shell region by a factor of 11.3. The results demonstrate that large CdSe/ZnS quantum dots with a small CdSe core have the necessary characteristics for efficient exciton dissociation and generation of free charge carriers.

The photophysics of charge-transfer and recombination mechanisms in a heterojunction structure of CdSe/CdS/Au quantum dots (QDs) are studied by temperature-dependent steady-state photoluminescence (PL) and time-resolved PL (TRPL). We manipulate the charge transfer from core to shell surface by varying the tunneling barrier height resulting from temperature variation and the barrier width resulting from shell thickness variation. The charge-transfer process, which can be described by a tunneling transmission model, is manifested by two competitive recombination processes, an intrinsic exciton emission and a trap emission in the near-infrared (NIR) range. Our study establishes the photophysics foundation for the core/shell/metal application in photocatalyst and optoelectronics.

The demand for clean energy will require the design of nanostructure-based light-harvesting assemblies for the conversion of solar energy into chemical energy (solar fuels) and electrical energy (solar cells). Semiconductor nanocrystals serve as the building blocks for designing next generation solar cells, and metal chalcogenides (e.g., CdS, CdSe, PbS, and PbSe) are particularly useful for harnessing size-dependent optical and electronic properties in these nanostructures. This Account focuses on photoinduced electron transfer processes in quantum dot sensitized solar cells (QDSCs) and discusses strategies to overcome the limitations of various interfacial electron transfer processes. The heterojunction of two semiconductor nanocrystals with matched band energies (e.g., TiO(2) and CdSe) facilitates charge separation. The rate at which these separated charge carriers are driven toward opposing electrodes is a major factor that dictates the overall photocurrent generation efficiency. The hole transfer at the semiconductor remains a major bottleneck in QDSCs. For example, the rate constant for hole transfer is 2-3 orders of magnitude lower than the electron injection from excited CdSe into oxide (e.g., TiO(2)) semiconductor. Disparity between the electron and hole scavenging rate leads to further accumulation of holes within the CdSe QD and increases the rate of electron-hole recombination. To overcome the losses due to charge recombination processes at the interface, researchers need to accelerate electron and hole transport. The power conversion efficiency for liquid junction and solid state quantum dot solar cells, which is in the range of 5-6%, represents a significant advance toward effective utilization of nanomaterials for solar cells. The design of new semiconductor architectures could address many of the issues related to modulation of various charge transfer steps. With the resolution of those problems, the efficiencies of QDSCs could approach those of dye sensitized solar cells (DSSC) and organic photovoltaics.

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Hierarchical ZnO nanosheets branched TiO₂ non-woven fabric film is used as efficient photoanode for QDSSCs.

Water-soluble, single-crystalline, and amine-functionalized graphene quantum dots (GQDs) with absorption edge at ∼490 nm were synthesized by a molecular fusion method, and stably deposited onto anatase TiO2 nanoparticles under hydrothermal conditions. The effective incorporation of the GQDs extends the light absorption of the TiO2 nanoparticles from UV to a wide visible region. Moreover, amine-functionalized GQD–TiO2 heterojunctions can absorb more O2 than pure TiO2, which can generate more ·O2 species for MO degradation. Accordingly, the heterojunctions exhibit much higher photocatalytic performance for degrading methyl orange (MO) under visible-light irradiation than TiO2 alone. At optimum GQD content (1.0 wt %), an apparent MO decomposition rate constant is 15 times higher than that of TiO2 alone, and photocurrent intensity in response to visible-light excitation increases by 9 times. Compared with conventional sensitization by toxic, photounstable quantum dots such as CdSe QDs, the sensitization by environmentally friendly GQDs shows higher visible-light photocatalytic activity and higher cycling stability. Monodispersed QD-based heterojunctions can effectively inhibit the fast recombination of electron–hole pairs of GQDs with a large exciton binding energy. The photogenerated electron transfer, energy-band-matching mechanism of GQD/TiO2, and possible MO decomposition pathways under visible-light irradiation are proposed.

The narrow bandgap semiconductor Sb2S3 has been extensively utilized in solar cells for its impressive light absorption coefficient and carrier mobility. However, a large number of deep energy defects provide carrier recombination sites that limit its photoelectrochemical (PEC) performance. In this study, Sb2S3 nanorods (NRs) and CdSe quantum dots (QDs) were successfully prepared, and Sb2S3/CdSe-annealed quasi-one-dimensional heterojunction photoanodes with the S-scheme were constructed. The Sb2S3/CdSe-annealed composite photoelectrode achieves a 12 times increase in photocurrent density compared to the 0.4 mA cm–2 of the Sb2S3 photoelectrode. Overall, the synergistic effect of the [hk1]-oriented Sb2S3 NRs and the S-scheme heterojunction promotes carrier transport, achieving a spatial separation of photogenerated carriers and efficiently weakening the recombination of the electron–hole pair. This study provides a simple method for the low-cost development of Sb2S3 NRs with [hk1] orientation and an idea to enhance its development in the PEC water splitting direction.

We report on bulk-heterojunction hybrid solar cells based on blends of non-ligand-exchanged CdSe quantum dots (QDs) and the conjugated polymer poly(3-hexylthiophene) with improved power conversion efficiencies of about 2% under AM1.5G illumination after spectral mismatch correction. This is the highest reported value for a spherical CdSe QD based photovoltaic device. After synthesis, the CdSe QDs are treated by a simple and fast acid-assisted washing procedure, which has been identified as a crucial factor in enhancing the device performance. A simple model of a reduced ligand sphere is proposed explaining the power conversion efficiency improvement.

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Solar energy conversion, particularly solar-driven chemical fuel formation, has been intensely studied in the past decades as a potential approach for renewable energy generation. Efficient solar-to-fuel conversion requires artificial photosynthetic systems with strong light absorption, long-lived charge separation and efficient catalysis. Colloidal quantum confined nanoheterostructures have emerged as promising materials for this application because of the ability to tailor their properties through size, shape and composition. In particular, colloidal one-dimensional (1D) semiconductor nanorods (NRs) offer the opportunity to simultaneously maintain quantum confinement in radial dimensions for tunable light absorptions and bulk like carrier transport in the axial direction for long-distance charge separations. In addition, the versatile chemistry of colloidal NRs enables the formation of semiconductor heterojunctions (such as CdSe/CdS dot-in-rod NRs) to separate photogenerated electron-hole pairs and deposition of metallic domains to accept charges and catalyze redox reactions. In this review, we summarize research progress on colloidal NR heterostructures and their applications for solar energy conversion, emphasizing mechanistic insights into the working principle of these systems gained from spectroscopic studies. Following a brief overview of synthesis of various NRs and heterostructures, we introduce their electronic structures and dynamics of exciton and carrier transport and interfacial transfer. We discuss how these exciton and carrier dynamics are controlled by their structures and provide key mechanistic understanding on their photocatalytic performance, including the photo-reduction of a redox mediator (methyl viologen) and light driven H2 generation. We discuss the solar-driven H2 generation mechanism, key efficiency limiting steps, and potential approaches for rational improvement in semiconductor NR/metal heterostructures (such as Pt tipped CdSe@CdS dot-in-rod NRs). Finally, we conclude by pointing out challenges to be addressed in future research.

PCE is improved owing to the enhanced <italic>J</italic>_SC, resulting from larger light harvesting and higher charge generation.

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Highly photoluminescent CdSe/CdTe/ZnSe type-II/type-I composite nanocrystals, both dot- and peanut-shaped, were prepared via the modified successive ionic layer adsorption and reaction (SILAR) techniques, straight SILAR for peanut-shaped ones and SILAR coupled with thermal-cycling (SILAR-TC) for dot-shaped ones. The CdSe/CdTe type-II heterojunction offered the nanocrystals with near-infrared emission and the CdTe/ZnSe type-I heterojunction helped to confine the photogenerated charges away from the ligands and solution environment. This structural feature makes the photoluminescence quantum yield of the CdSe/CdTe/ZnSe core/shell/shell type-II/type-I dots that have a uniformly grown ZnSe shell retain as high as 60% after replacing the original amine ligands with mercaptopropionic acid (MPA). Conversely, the emission of the corresponding CdSe/CdTe core/shell dots (CdSe/CdTe/ZnSe composite peanuts) was completely (almost completely) quenched by the same ligand treatment. The emission properties of the MPA-coated CdSe/CdTe/ZnSe core/shell/shell dots were stable in water in the buffer solutions with their pH in a range between about 5 and 9.

Advanced architectures are required to further improve the performance of colloidal PbS heterojunction quantum dot solar cells. Here, we introduce a CdI₂-treated CdSe quantum dot buffer layer at the junction between ZnO nanoparticles and PbS quantum dots in the solar cells. We exploit the surface- and size-tunable electronic properties of the CdSe quantum dots to optimize its carrier concentration and energy band alignment in the heterojunction. We combine optical, electrical, and analytical measurements to show that the CdSe quantum dot buffer layer suppresses interface recombination and contributes additional photogenerated carriers, increasing the open-circuit voltage and short-circuit current of PbS quantum dot solar cells, leading to a 25% increase in solar power conversion efficiency.

Type II CdSe/CdTe core/shell nanocrystals with a dot shape were synthesized using a modified SILAR technique that incorporates cycling of the reaction temperature (thermal cycling). Conversely, experimental results revealed that the standard SILAR alone produced type II core/shell nanocrystals in a peanut shape (1D). Despite their differences in shape, the optical properties observed for the type II dot- and peanut-shaped core/shell nanocrystals were similar. The dot-shaped nanocrystals were confirmed as core/shell structures with an abrupt type II heterojunction within the experimental accuracy, and the peanut-shaped ones were found to be consistent with CdSe and CdTe separated on the two ends of the rods. Similar techniques were used for the synthesis of CdS/CdSe/CdTe type II colloidal quantum well heterostructures with dot and peanut shapes. For these type II colloidal quantum well structures, the PL peak positions were shown to be readily tunable by varying the CdSe and CdTe shell thickness, something not typically seen for the quantum dots. The PL quantum yield of these nanocrystals were found to range between 30 and 60%.

To elucidate minority carrier transport in CdSe quantum dot films, a detailed DLTS study on TiO2/CdSe quantum dot heterojunctions is performed. Long transient times are found, which are related to tunneling instead of the thermal emission of electrons. Surprisingly, the transient times increase with increasing temperature, which is possibly related to thermal expansion of the tunnel barrier width between quantum dots. This effect can give rise to unexpected behavior of quantum dot devices.

Colloidal CdSe quantum dots (QDs) are suitable as electron acceptors in polymer/nanoparticle bulk heterojunction hybrid solar cells. For this application, a thick organic ligand shell which is typically surrounding the QDs after synthesis needs to be removed. Ligand exchange with pyridine is the most widely used method for this purpose. Although this approach is already 15 years old, detailed studies on the effectiveness of ligand exchange with pyridine for solar cell applications are still missing. In the present work hybrid solar cells were prepared from CdSe QDs initially capped with oleic acid (OA), and the impact of single and multiple pyridine treatment was thoroughly investigated. NMR was applied to determine the composition of the ligand shell as well as to distinguish the bound and free ligands before and after ligand exchange. It is shown that after a single pyridine treatment some amount of OA is still present in the samples. By using thermal gravimetric analysis (TGA) we could obtain also quantitative information about the effectiveness of subsequent pyridine treatments. In a series of one-, two-, and threefold ligand exchange, the estimated surface coverage by OA decreased from 26% to 12%, whereas that of pyridine increased from 54% to 80%. Laboratory solar cells with pyridine-capped CdSe QDs and poly(3-hexylthiophene) (P3HT) were characterized by current−voltage (I−V) measurements, and in order to get deeper insight into charge carrier generation and recombination processes, CdSe/P3HT blends were studied by light-induced electron spin resonance (l-ESR). Although repeated pyridine treatment was found to have a beneficial effect in the sense that more complete ligand exchange was achieved, which in turn enabled more efficient charge transfer, the performance of the solar cells was found to be reduced. This fact correlates with increased aggregation tendency of repeatedly pyridine-treated particles, negatively influencing the morphology of the blends, as well as with a larger amount of surface defects in particles stabilized by the weak pyridine ligand shell.

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A series of highly efficient semiconductor nanocrystal (NC) photocatalysts have been synthesized by growing wurtzite-ZnO tetrahedrons around pre-formed CdS, CdSe, and CdTe quantum dots (QDs). The resulting contact between two small but high-quality crystals creates novel CdX/ZnO heterostructured semiconductor nanocrystals (HSNCs) with extensive type-II nanojunctions that exhibit more efficient photocatalytic decomposition of aqueous organic molecules under UV irradiation. Catalytic testing and characterization indicate that catalytic activity increases as a result of a combination of both the intrinsic chemistry of the chalcogenide anions and the heterojunction structure. Atomic probe tomography (APT) is employed for the first time to probe the spatial characteristics of the nanojunction between cadmium chalcogenide and ZnO crystalline phases, which reveals various degrees of ion exchange between the two crystals to relax large lattice mismatches. In the most extreme case, total encapsulation of CdTe by ZnO as a result of interfacial alloying is observed, with the expected advantage of facilitating hole transport for enhanced exciton separation during catalysis.

&lt;p&gt;Charge transfer in semiconductor heterojunctions is largely governed by the offset in the energy levels of the constituent materials. Unfortunately, literature values for such energy level offsets vary widely and are usually based on energy levels of the individual materials rather than of actual heterojunctions. Here we present a new method to determine absolute energy levels and energy level offsets in situ for films containing CdSe and PbSe quantum dots. Using spectroelectrochemistry, we find a type I offset at the CdSe-PbSe heterojunction. Whereas the energy level offset follows the expected size-dependent trend, the absolute positions of the 1S&lt;sub&gt;e&lt;/sub&gt; level in the individual CdSe or PbSe quantum dots does not. This level varies by more than 0.5 eV, depending on film composition and surface defect concentration. Rather than extrapolating energy level offsets from measurements on pure CdSe or PbSe quantum-dot films, we suggest measuring energy level offsets in heterojunctions in situ.&lt;/p&gt;

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Semiconductor-metal nanoheterostructures, such as CdSe/CdS dot-in-rod nanorods with a Pt tip at one end (or CdSe/CdS-Pt), are promising materials for solar-to-fuel conversion because they allow rational integration of a light absorber, hole acceptor, and electron acceptor or catalyst in an all-inorganic triadic heterostructure as well as systematic control of relative energetics and spatial arrangement of the functional components. To provide design principles of such triadic nanorods, we examined the photocatalytic H2 generation quantum efficiency and the rates of elementary charge separation and recombination steps of CdSe/CdS-Pt and CdS-Pt nanorods. We showed that the steady-state H2 generation quantum efficiencies (QEs) depended sensitively on the electron donors and the nanorods. Using ultrafast transient absorption spectroscopy, we determined that the electron transfer efficiencies to the Pt tip were near unity for both CdS and CdSe/CdS nanorods. Hole transfer rates to the electron donor, measured by time-resolved fluorescence decay, were positively correlated with the steady-state H2 generation QEs. These results suggest that hole transfer is a key efficiency-limiting step. These insights provide possible ways for optimizing the hole transfer step to achieve efficient solar-to-fuel conversion in semiconductor-metal nanostructures.

We demonstrated that the aspect ratio (AR)-tunable CdSe/CdS dot-in-rod (DiR) nanostructures with quasi-type-II band structure were successively synthesized using the hot injection method. When the AR of CdSe/CdS DiR was tuned from 10 to 37, the exciton localization efficiency along the longitudinal CdS rod shell decreased from 57.9 to 15.1%, resulting in a 5-fold improvement in the efficiency of photocatalytic hydrogen (H₂) evolution. The optimal CdSe/CdS DiR exhibited the highest H₂ evolution rate of 2.11 mmol·g^-1·h^-1 at an AR of 29 without any cocatalyst assistance. In situ transient absorption spectroscopy was employed to investigate the interfacial charge carrier dynamics of CdSe/CdS DiR during practical photocatalytic H₂ evolution. The findings indicated that the half-life of delocalized electrons on the conduction band along the longitudinal CdS rod shell increases from 11.5 to 20.1 μs as the AR increased, demonstrating that the AR-dependent charge carrier dynamics significantly influences the photoactivity of CdSe/CdS DiR. This study provides valuable and novel insights into the tunability of charge carrier dynamics through AR manipulation in one-dimensional semiconductor nano-heterostructures for solar fuel generation.

Recent years have witnessed quite a number of worldwide efforts for fabricating CdSe/TiO2 nanotube arrays (CdSe/TNTAs) nanocomposites; however, the construction of a well-defined CdSe/TNTAs binary nanostructure for versatile photocatalytic and photoelectrochemical applications still poses a big challenge. In this work, a hierarchically ordered CdSe/nanoporous TiO2 nanotube arrays (CdSe/NP-TNTAs) hybrid nanostructure was fabricated through a facile electrochemical deposition strategy. The combined structural and morphological characterizations show that the CdSe ingredients, consisting of clusters of quantum dots, were uniformly assembled on the inner and outer surfaces of the NP-TNTAs framework. It was demonstrated that the CdSe/NP-TNTAs heterostructure could be utilized as an efficient photoanode for photoelectrochemical water splitting; moreover, it could be used as a multifunctional photocatalyst for photoredox applications, including the photocatalytic oxidation of organic dye pollutants and the selective reduction of aromatic nitro compounds under visible light irradiation. Furthermore, photoelectrochemical and photocatalytic mechanisms over the CdSe/NP-TNTAs heterostructure were elucidated. In addition, the predominant active species during the photocatalytic process were systematically explored and unequivocally determined. It is hoped that this work could promote further interest in the fabrication of various one dimensional NP-TNTAs-based composite materials and their applications to photoelectrochemical water splitting and photocatalytic selective redox applications.

Electrostatic force microscopy is used to study light-induced charging in single hybrid Au-CdSe nanodumbbells. Upon illumination, nanodumbbells show negative charging, which is in contrast with CdSe rods and Au particles that show positive charging. This different behavior is attributed to charge separation in the nanodumbbells, where after excitation the electron is transferred to the gold tips and the hole is subsequently filled through tunneling interactions with the substrate. The process of light-induced charge separation at the metal-semiconductor interface is key for the photocatalytic activity of such hybrid metal-semiconductor nanostructures.

CdSe/CdS core/shell semiconductor nanorods (NR) with rod-in-rod morphology offer new strategies for designing highly emissive nanostructures. The interplay between energetically matched semiconductors results in enhanced emission from the CdSe core. In order to further evaluate the cooperative role of these two semiconductors in a core/shell geometry, we have probed the photoinduced charge transfer between CdSe/CdS core/shell semiconductor NR and methyl viologen (MV2+). The quenching of the emission by the electron acceptor, MV2+, as well as the production of electron transfer product MV•+ depends on the aspect ratio (l/w) of the NR thus pointing out the role of CdS shell in determining the overall photocatalytic efficiency. Transient absorption measurements show that the presence of MV2+ influences only the bleaching recovery of the CdS shell and not of the CdSe core recovery. Thus, optimization of shell aspect ratio plays a crucial role in maximizing the efficiency of this photocatalytic system.

Plasmonic heteronanostructures in semiconductor type display extraordinary photocatalytic efficiency induced by the plasmonic energy that operates in the Ag@CdSe-rGO hybrid ternary composites. The obtained plasmonic photocatalysts in nanoscale were fabricated by using a one-step hydrothermal method, during which the in situ nucleation of Ag@CdSe core-shell nanoparticles and the reduction of GO to rGO occurred simultaneously. Three different roles of Ag core and the junction of synergistic properties arising from the introduced rGO jointly enhanced the optical properties of CdSe. Localized plasmon resonance (LPR) effects of plasmonic Ag contribute to the separation of photogenerated e(-)/h(+) pairs via the electrons and resonant energy transfer. Electrochemical investigations have further confirmed the enhanced separation of the photogenerated e(-)/h(+) pairs. From comparative photocatalytic experiments of Ag@CdSe-rGO and Ag/CdSe-rGO, the plasmonic effect of the Ag core in the Ag@CdSe-rGO nanostructure serves to prolong the charge separation under visible light beyond common attached trimers.

In the present work, we focused on geometrical (single- or double-tipped) and compositional (Pt or Au) variations of active metal components in a well-defined CdSe nanorod system. These colloidal nanostructures were employed for photocatalytic hydrogen generation from water under the identical reaction conditions with visible light irradiation. The catalysts exhibited significant dependency of the catalytic activity, specifically on the catalyst geometry and the choice of the metal tips, determined by the energetic consideration of electron transfer to the metal tips and hole transfer to the sacrificial reagents on the CdSe nanorods.

We synthesized various nanocrystal (e.g., CdS, CdSe, and Cu2S)-carbon nanotube (NC-CNT) and NC-TiO2 hybrid nanostructures using the solvothermal method and compared their photocatalytic ability toward the visible-light-driven degradation of methylene blue (MB) dye. The free CdS NCs exhibited higher degradation efficiency than the CdSe and Cu2S NCs. The photocatalytic abilities of the NCs were found to determine the relative degradation efficiency of their CNT and TiO2 hybrid nanostructures. These results suggest that the oxidative N-demethylation degradation involves the transfer of holes from the NCs to MB. The hybridization of the NCs with the TiO2 NCs and CNTs enhances the oxidative degradation rate to the same extent, suggesting that the interfacial electron transfer process from the NCs to the attached CNTs (or TiO2), which retards the recombination of the electrons and holes, is comparable for both hybrid nanostructures.

Here, the photocatalytic CO₂ reduction reaction (CO₂ RR) 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 Cu₂ O 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-Cu₂ O 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 H₂ O reduction are indispensable. Under the optimum condition, the Au/CdSe-Cu₂ O 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.

A new composite photocatalyst CdSe-sensitized TiO2 nanorod array (CdSe/TiO2 NRA) grown on fluorine-doped tin oxide (FTO) coated with a TiO2:Yb3+,Er3+ thin film was constructed. The TiO2:Yb3+,Er3+ thin film acted as a medium for converting near-IR light to visible light via an upconversion process and reduced electron–hole recombination. TiO2 NRAs with high specific surface areas and well-aligned nanostructures can provide a fast transfer pathway for photogenerated electrons. The CdSe/TiO2 NRAs extended the optical response from the ultraviolet to visible region, which can generate more electron–hole pairs. The composite photocatalyst TiO2 NRAs/TiO2:Yb3+,Er3+ exhibited excellent photocatalytic activity toward the degradation of Rhodamine B. The composite structure can not only extend the absorption of TiO2 but also consequently reduce electron–hole recombination, which improve the photocatalytic efficiency of the composite photocatalyst. Moreover, the photocatalyst grown on FTO substrates directly makes it possible to collect and recycle.

Charge separation and charge transfer across interfaces are key aspects in the design of efficient photocatalysts for solar energy conversion. In this study, we investigate the hydrogen generating capabilities and underlying photophysics of nanostructured photocatalysts based on CdSe nanowires (NWs). Systems studied include CdSe, CdSe/CdS core/shell nanowires and their Pt nanoparticle-decorated counterparts. Femtosecond transient differential absorption measurements reveal how semiconductor/semiconductor and metal/semiconductor heterojunctions affect the charge separation and hydrogen generation efficiencies of these hybrid photocatalysts. In turn, we unravel the role of surface passivation, charge separation at semiconductor interfaces and charge transfer to metal co-catalysts in determining photocatalytic H2 generation efficiencies. This allows us to rationalize why Pt nanoparticle decorated CdSe/CdS NWs, a double heterojunction system, performs best with H2 generation rates of ∼434.29 ± 27.40 μmol h(-1) g(-1) under UV/Visible irradiation. In particular, we conclude that the CdS shell of this double heterojunction system serves two purposes. The first is to passivate CdSe NW surface defects, leading to long-lived charges at the CdSe/CdS interface capable of carrying out reduction chemistries. Upon photoexcitation, we also find that CdS selectively injects charges into Pt NPs, enabling simultaneous reduction chemistries at the Pt NP/solvent interface. Pt nanoparticle decorated CdSe/CdS NWs thus enable reduction chemistries at not one, but rather two interfaces, taking advantage of each junction's optimal catalytic activities.

Semiconductor-metal hybrid nanostructures are one of the best model catalysts for understanding photocatalytic hydrogen generation. To investigate the optimal structure of metal cocatalysts, metal-CdSe-metal nanodumbbells were synthesized with three distinct sets of metal tips, Pt-CdSe-Pt, Au-CdSe-Au, and Au-CdSe-Pt. Photoelectrochemical responses and transient absorption spectra showed that the competition between the charge recombination at the metal-CdSe interface and the water reduction on the metal surface is a detrimental factor for the apparent hydrogen evolution rate. For instance, a large recombination rate (k_rec) at the Pt-CdSe interface limits the quantum yield of hydrogen generation despite a superior water reduction rate (k_WR) on the Pt surface. To suppress the recombination process, Pt was selectively deposited onto the Au tips of Au-CdSe-Au nanodumbbells in which the k_rec was diminished at the Au-CdSe interface, and the large k_WR was maintained on the Pt surface. As a result, the optimal structure of the Pt-coated Au-CdSe-Au nanodumbbells reached a quantum yield of 4.84%. These findings successfully demonstrate that the rational design of a metal cocatalyst and metal-semiconductor interface can additionally enhance the catalytic performance of the photochemical hydrogen generation reactions.

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List of Contributors. Preface. Editor Biography. PART ONE-FUNDAMENTALS, MODELING, AND EXPERIMENTAL INVESTIGATION OF PHOTOCATALYTIC REACTIONS FOR DIRECT SOLAR HYDROGEN GENERATION. 1 Solar Hydrogen Production by Photoelectrochemical Water Splitting: The Promise and Challenge (Eric L. Miller). 1.1 Introduction. 1.2 Hydrogen or Hype? 1.3 Solar Pathways to Hydrogen. 1.4 Photoelectrochemical Water-Splitting. 1.5 The Semiconductor/Electrolyte Interface. 1.6 Photoelectrode Implementations. 1.7 The PEC Challenge. 1.8 Facing the Challenge: Current PEC Materials Research. Acknowledgments. References. 2 Modeling and Simulation of Photocatalytic Reactions at TiO2 Surfaces (Hideyuki Kamisaka and Koichi Yamashita). 2.1 Importance of Theoretical Studies on TiO2 Systems. 2.2 Doped TiO2 Systems: Carbon and Niobium Doping. 2.3 Surface Hydroxyl Groups and the Photoinduced Hydrophilicity of TiO2. Conversion. 2.4 Dye-Sensitized Solar Cells. 2.5 Future Directions: Ab Initio Simulations and the Local Excited States on TiO2. Acknowledgments. References. 3 Photocatalytic Reactions on Model Single Crystal TiO2 Surfaces (G.I.N. Waterhouse and H. Idriss). 3.1 TiO2 Single-Crystal Surfaces. 3.2 Photoreactions Over Semiconductor Surfaces. 3.3 Ethanol Reactions Over TiO2(110) Surface. 3.4 Photocatalysis and Structure Sensitivity. 3.5 Hydrogen Production from Ethanol Over Au/TiO2 Catalysts. 3.6 Conclusions. References. 4 Fundamental Reactions on Rutile TiO2(110) Model Photocatalysts Studied by High-Resolution Scanning Tunneling Microscopy (Stefan Wendt, Ronnie T. Vang, and Flemming Besenbacher). 4.1 Introduction. 4.2 Geometric Structure and Defects of the Rutile TiO2 (110) Surface. 4.3 Reactions of Water with Oxygen Vacancies. 4.4 Splitting of Paired H Adatoms and Other Reactions Observed on Partly Water Covered TiO2(110). 4.5 O2 Dissociation and the Role of Ti Interstitials. 4.6 Intermediate Steps of the Reaction Between O2 and H Adatoms and the Role of Coadsorbed Water. 4.7 Bonding of Gold Nanoparticles on TiO2(110) in Different Oxidation States. 4.8 Summary and Outlook. References. PART TWO-ELECTRONIC STRUCTURE, ENERGETICS, AND TRANSPORT DYNAMICS OF PHOTOCATALYST NANOSTRUCTURES. 5 Electronic Structure Study of Nanostructured Transition Metal Oxides Using Soft X-Ray Spectroscopy (Jinghua Guo, Per-Anders Glans, Yi-Sheng Liu, and Chinglin Chang). 5.1 Introduction. 5.2 Soft X-Ray Spectroscopy. 5.3 Experiment Set-Up. 5.4 Results and Discussion. Acknowledgments. References. 6 X-ray and Electron Spectroscopy Studies of Oxide Semiconductors for Photoelectrochemical Hydrogen Production (Clemens Heske, Lothar Weinhardt, and Marcus B€ar). 6.1 Introduction. 6.2 Soft X-Ray and Electron Spectroscopies. 6.3 Electronic Surface-Level Positions of WO3 Thin Films. 6.4 Soft X-Ray Spectroscopy of ZnO:Zn3N2 Thin Films. 6.5 In Situ Soft X-Ray Spectroscopy: A Brief Outlook. 6.6 Summary. Acknowledgments. References. 7 Applications of X-Ray Transient Absorption Spectroscopy in Photocatalysis for Hydrogen Generation (Lin X. Chen). 7.1 Introduction. 7.2 X-Ray Transient Absorption Spectroscopy (XTA). 7.3 Tracking Electronic and Nuclear Configurations in Photoexcited Metalloporphyrins. 7.4 Tracking Metal-Center Oxidation States in the MLCT State of Metal Complexes. 7.5 Tracking Transient Metal Oxidation States During Hydrogen Generation. 7.6 Prospects and Challenges in Future Studies. Acknowledgments. References. 8 Fourier-Transform Infrared and Raman Spectroscopy of Pure and Doped TiO2 Photocatalysts (Lars Osterlund). 8.1 Introduction. 8.2 Vibrational Spectroscopy on TiO2 Photocatalysts: Experimental Considerations. 8.3 Raman Spectroscopy of Pure and Doped TiO2 Nanoparticles. 8.4 Gas-Solid Photocatalytic Reactions Probed by FTIR Spectroscopy. 8.5 Model Gas-Solid Reactions on Pure and Doped TiO2 Nanoparticles Studied by FTIR Spectroscopy. 8.6 Summary and Concluding Remarks. Acknowledgments. References. 9 Interfacial Electron Transfer Reactions in CdS Quantum Dot Sensitized TiO2 Nanocrystalline Electrodes (Yasuhiro Tachibana). 9.1 Introduction. 9.2 Nanomaterials. 9.3 Transient Absorption Spectroscopy. 9.4 Controlling Interfacial Electron Transfer Reactions by Nanomaterial Design. 9.5 Application of QD-Sensitized Metal-Oxide Semiconductors to Solar Hydrogen Production. 9.6 Conclusion. Acknowledgments. References. PART THREE-DEVELOPMENT OF ADVANCED NANOSTRUCTURES FOR EFFICIENT SOLAR HYDROGEN PRODUCTION FROM CLASSICAL .LARGE BANDGAP SEMICONDUCTORS. 10 Ordered Titanium Dioxide Nanotubular Arrays as Photoanodes for Hydrogen Generation (M. Misra and K.S. Raja). 10.1 Introduction. 10.2 Crystal Structure of TiO2. References. 11 Electrodeposition of Nanostructured ZnO Films and Their Photoelectrochemical Properties (Torsten Oekermann). 11.1 Introduction. 11.2 Fundamentals of Electrochemical Deposition. 11.3 Electrodeposition of Metal Oxides and Other Compounds. 11.4 Electrodeposition of Zinc Oxide. 11.5 Electrodeposition of One- and Two-Dimensional ZnO Nanostructures. 11.6 Use of Additives in ZnO Electrodeposition. 11.7 Photoelectrochemical and Photovoltaic Properties. 11.8 Photocatalytic Properties. 11.9 Outlook. References. 12 Nanostructured Thin-Film WO3 Photoanodes for Solar Water and Sea-Water Splitting (Bruce D. Alexander and Jan Augustynski). 12.1 Historical Context. 12.2 Macrocrystalline WO3 Films. 12.3 Limitations of Macroscopic WO3. 12.4 Nanostructured Films. 12.5 Tailoring WO3 Films Through a Modified Chimie Douce Synthetic Route. 12.6 Surface Reactions at Nanocrystalline WO3 Electrodes. 12.7 Conclusions and Outlook. References. 13 Nanostructured a-Fe2O3 in PEC Generation of Hydrogen (Vibha R. Satsangi, Sahab Dass, and Rohit Shrivastav). 13.1 Introduction. 13.2 a-Fe2O3. 13.3 Nanostructured a-Fe2O3 Photoelectrodes. 13.5 Efficiency and Hydrogen Production. 13.6 Concluding Remarks. Acknowledgments. References. PART FOUR-NEW DESIGN AND APPROACHES TO BANDGAP PROFILING AND VISIBLE-LIGHT-ACTIVE NANOSTRUCTURES. 14 Photoelectrocatalyst Discovery Using High-Throughput Methods and Combinatorial Chemistry (Alan Kleiman-Shwarsctein, Peng Zhang, Yongsheng Hu, and Eric W. McFarland). 14.1 Introduction. 14.2 The Use of High-Throughput and Combinatorial Methods for the Discovery and Optimization of Photoelectrocatalyst Material Systems. 14.3 Practical Methods of High-Throughput Synthesis of Photoelectrocatalysts. 14.4 Photocatalyst Screening and Characterization. 14.5 Specific Examples of High-Throughput Methodology Applied to Photoelectrocatalysts. 14.6 Summary and Outlook. References. 15 Multidimensional Nanostructures for Solar Water Splitting: Synthesis, Properties, and Applications (Abraham Wolcott and Jin Z. Zhang). 15.1 Motivation for Developing Metal-Oxide Nanostructures. 15.2 Colloidal Methods for 0D Metal-Oxide Nanoparticle Synthesis. 15.3 1D Metal-Oxide Nanostructures. 15.4 2D Metal-Oxide Nanostructures. 15.5 Conclusion. Acknowledgments. References. 16 Nanoparticle-Assembled Catalysts for Photochemical Water Splitting (Frank E. Osterloh). 16.1 Introduction. 16.2 Two-Component Catalysts. 16.3 CdSe Nanoribbons as a Quantum-Confined Water-Splitting Catalyst. 16.4 Conclusion and Outlook. Acknowledgment. References. 17 Quantum-Confined Visible-Light-Active Metal-Oxide Nanostructures for Direct Solar-to-Hydrogen Generation (Lionel Vayssieres). 17.1 Introduction. 17.2 Design of Advanced Semiconductor Nanostructures by Cost-Effective Technique. 17.3 Quantum Confinement Effects for Photovoltaics and Solar Hydrogen Generation. 17.4 Novel Cost-Effective Visible-Light-Active (Hetero)Nanostructures for Solar Hydrogen Generation. 17.5 Conclusion and Perspectives. References. 18 Effects of Metal-Ion Doping, Removal and Exchange on Photocatalytic Activity of Metal Oxides and Nitrides for Overall Water Splitting (Yasunobu Inoue). 18.1 Introduction. 18.2 Experimental Procedures. 18.3 Effects of Metal Ion Doping. 18.4 Effects of Metal-Ion Removal. 18.5 Effects of Metal-Ion Exchange on Photocatalysis. 18.6 Effects of Zn Addition to Indate and Stannate. 18.7 Conclusions. Acknowledgments. References. 19 Supramolecular Complexes as Photoinitiated Electron Collectors: Applications in Solar Hydrogen Production (Shamindri M. Arachchige and Karen J. Brewer). 19.1 Introduction. 19.2 Supramolecular Complexes for Photoinitiated Electron Collection. 19.3 Conclusions. List of Abbreviations. Acknowledgments. References. PART FIVE-NEW DEVICES FOR SOLAR THERMAL HYDROGEN GENERATION. 20 Novel Monolithic Reactors for Solar Thermochemical Water Splitting (Athanasios G. Konstandopoulos and Souzana Lorentzou). 20.1 Introduction. 20.2 Solar Hydrogen Production. 20.3 HYDROSOL Reactor. 20.4 HYDROSOL Process. 20.5 Conclusions. Acknowledgments. References. 21 Solar Thermal and Efficient Solar Thermal/Electrochemical Photo Hydrogen Generation (Stuart Licht). 21.1 Comparison of Solar Hydrogen Processes. 21.2 STEP (Solar Thermal Electrochemical Photo) Generation of H2. 21.3 STEP Theory. 21.4 STEP Experiment: Efficient Solar Water Splitting. 21.5 NonHybrid Solar Thermal Processes. 21.6 Conclusions. References. Index

Abstract Metal chalcogenide‐based semiconductor nanostructures are promising candidates for photocatalytic or photoelectrocatalytic hydrogen generation. In order to protect CdSe from photocorrosion, a layer of TiO 2 wrapped (shell) onto CdSe (core) nanocapsule via the post‐synthesis process. The morphology studies confirm that a thin crystalline TiO 2 shell (3–8 nm) wrapped in all the three directions onto CdSe core and thickness of the shell can be controlled through modulating the titania precursor concentration. The feasibility of pristine CdSe nanocapsules and CdSe@TiO 2 in transforming visible light to hydrogen conversion was tested through photocatalysis reaction. The CdSe@TiO 2 nanocapsules generated a four‐fold high rate of hydrogen gas (21 mmol.h −1 .g −1 cat) than pristine CdSe. In order to understand the role of shell@core, we have studied the photoelectrochemical and impedance analysis. The CdSe@TiO 2 nanocapsules showed higher photoelectric current generation and lower charge transfer resistance at electrode/electrolyte interfaces compared to pristine CdSe. These studies endorse that chemically synthesized crystalline TiO 2 shell played a multifunctional role in (a) surface passivation from photocorrosion, (b) promoting photocharge carrier separation via tunneling process between CdSe and TiO 2 interface. As a result, CdSe@TiO 2 nanocapsules showed a high conversion efficiency of 12.9 % under visible light irradiation (328 mW.cm −2 ) and a TOF of 0.05018 s −1 .

Constructing photocatalysts to promote hydrogen evolution and carbon dioxide photoreduction into solar fuels is of vital importance. The design and establishment of an S-scheme heterojunction system is one of the most feasible approaches to facilitate the separation and transfer of photogenerated charge carriers and obtain powerful photoredox capabilities for boosting photocatalytic performance. Herein, a zero-dimensional/one-dimensional S-scheme heterojunction composed of CdSe quantum dots and polymeric carbon nitride nanorods (CdSe/CN) is created and constructed via a linker-assisted hybridization approach. The CdSe/CN composites exhibit superior photocatalytic activity in water splitting and promoted carbon dioxide conversion performance compared with CN nanorods and CdSe quantum dots. The best efficiency in photocatalytic water splitting (10.2% apparent quantum yield at 420 nm irradiation, 20.1 mmol g^-1 h^-1 hydrogen evolution rate) and CO₂ reduction (0.77 mmol g^-1 h^-1 CO production rate) was achieved by 5%CdSe/CN composites. The significantly improved photocatalytic reactivity of CdSe/CN composites primarily originates from the emergence of an internal electric field in the zero-dimensional/one-dimensional S-scheme heterojunction, which could greatly improve the photoinduced charge-carrier separation. This work underlines the possibility of employing polymeric carbon nitride nanostructures as appropriate platforms to establish highly active S-scheme heterojunction photocatalysts for solar fuel production.

CdSe@CdS dot-in-rod nanostructures tipped with AuPt bimetallic nanoparticles as cocatalyst show increased photon-to-hydrogen conversion efficiency compared to their analogues with pure Au or Pt tips. The underlying charge-separation and recombination processes are investigated by time-resolved transient absorption spectroscopy, to unravel whether the observed enhancement of photocatalytic activity is due to charge-separation/recombination properties of the system or to higher reactivity for proton reduction at the surface of the metal nanoparticle. We find that in the catalytically active Pt- and AuPt-functionalized structures charge separation occurs with similar time constants (Pt 3.5, 35, and 49 ps; AuPt 2.6, 31, and 66 ps), and the charge-separated state shows a lifetime of ∼20 μs in both cases. Hence, these processes should not be regarded as a source of the increased catalytic efficiency in the AuPt-functionalized nanorods. The results indicate that the proton reduction at the metal nanoparticle surface itself determines the overall efficiency.

We study plasmonic control of photocatalytic properties of metal oxides and the ways they influence interaction of quantum dots with metallic nanostructures. For this, gold nanostructures are coated with ultrathin layers of metal oxides (Al, Cu, Cr, or Ti oxide) and then covered with CdSe/ZnS quantum dots. The results show how the photocatalytic properties of such metal oxides are renormalized by plasmon near fields. In the cases of Al, Cr, and Ti oxides, the results mostly indicate the direct impact of plasmon fields via enhancement of optical excitations of the quantum dots. For the case of Cu oxide, however, the outcomes are found to be quite unique. In the absence of the plasmonic structures, such an oxide (CuO) presents highly active photocatalytic processes, leading to complete annihilation of the quantum dot emission. In the presence of the metallic nanostructures, the emission of such quantum dots is revived, offering an ultrafast decay process (∼112 ps). These results indicate that in the case of CuO, the plasmonic metal oxide-induced photocatalytic processes include not only direct impact of plasmon near fields on the optical excitations of quantum dots but also the enhancement of interband transitions in CuO nanoparticles. The effects of energy transfer from quantum dots to metallic nanostructures and its equalization with Purcell effects on such processes are discussed.

We report on a photocatalytic system consisting of CdSe@CdS nanorods coated with a polydopamine (PDA) shell functionalized with molecular rhodium catalysts. The PDA shell was implemented to enhance the photostability of the photosensitizer, to act as a charge-transfer mediator between the nanorods and the catalyst, and to offer multiple options for stable covalent functionalization. This allows for spatial proximity and efficient shuttling of charges between the sensitizer and the reaction center. The activity of the photocatalytic system was demonstrated by light-driven reduction of nicotinamide adenine dinucleotide (NAD⁺) to its reduced form NADH. This work shows that PDA-coated nanostructures present an attractive platform for covalent attachment of reduction and oxidation reaction centers for photocatalytic applications.

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CdSe/CdTe heterojunction nanorods with type II staggered band offset can allow directional and efficient separation of photogenerated charge carriers. However, CdTe nanocrystals can often be easily oxidized even with postsynthesis processing in air, which can then lead to charge traps that negate the benefits of the type II band offset. Here, we introduce a simple ligand exchange method to replace the native ligands on CdSe/CdTe heterojunction nanorods with 1-octanethiol resulting in improved photoluminescence and good stability in air. Transient absorption measurements reveal that electron transfer from CdTe to CdSe remains efficient/fast (∼400 fs) despite the hole trapping nature of thiol ligands for CdSe. Absorption bleach arising from CdTe-to-CdSe electron transfer can be observed out to 1 μs even after days of storage in air, an order of magnitude longer than heterojunction nanorods with native ligands that are processed with anhydrous solvents under air-free conditions and kept air-free. This improved stability/robustness that preserves efficient charge separation translates to enhanced photocurrent generation especially with respect to contribution from photoexcitation of CdTe transitions.

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Utilization of hot carriers is very crucial in improving the efficiency of solar energy devices. In this work, we have fabricated an Sb₂Se₃/CdSe p-n heterojunction via a cation exchange method and investigated the possibility of hot electron transfer and relaxation pathways through ultrafast spectroscopy. The enhanced intensity of the CdSe hot excitonic (1P) bleach in the heterostructure system confirmed the hot electron transfer from Sb₂Se₃ to CdSe. Both the 1S and 1P signals are dynamically very slow in the heterosystem, validating this charge migration phenomenon. Interestingly, recovery of the 1P signal is much slower than that of 1S. This is very unusual as 1S is the lowest-energy state. This observation indicates the strength of hot electron transfer in this unique heterojunction, which helps in increasing the carrier lifetime in the hot state. Extended separation of charge carriers and enhanced hot carrier lifetime would be extremely helpful in extracting carriers and boost the performance of optoelectronic devices.

Colloidal cadmium chalcogenide nanosheets with atomically precise thickness of a few atomic layers and size of 10-100 nm are two-dimensional (2D) quantum well materials with strong and precise quantum confinement in the thickness direction. Despite their many advantageous properties, excitons in these and other 2D metal chalcogenide materials are short-lived due to large radiative and nonradiative recombination rates, hindering their applications as light harvesting and charge separation/transport materials for solar energy conversion. We showed that these problems could be overcome in type-II CdSe/CdTe core/crown heteronanosheets (with CdTe crown laterally extending on the CdSe nanosheet core). Photoluminesence excitation measurement revealed that nearly all excitons generated in the CdSe and CdTe domains localized to the CdSe/CdTe interface to form long-lived charge transfer excitons (with electrons in the CdSe domain and hole in the CdTe domain). By ultrafast transient absorption spectroscopy, we showed that the efficient exciton localization efficiency could be attributed to ultrafast exciton localization (0.64 ± 0.07 ps), which was facilitated by large in-plane exciton mobility in these 2D materials and competed effectively with exiton trapping at the CdSe or CdTe domains. The spatial separation of electrons and holes across the CdSe/CdTe heterojunction effectively suppressed radiative and nonradiative recombination processes, leading to a long-lived charge transfer exciton state with a half-life of ∼ 41.7 ± 2.5 ns, ∼ 30 times longer than core-only CdSe nanosheets.

To better understand the role nanoscale heterojunctions play in the photocatalytic generation of hydrogen, we have designed several model one-dimensional (1D) heterostructures based on CdSe nanowires (NWs). Specifically, CdSe/CdS core/shell NWs and Au nanoparticle (NP)-decorated core and core/shell NWs have been produced using facile solution chemistries. These systems enable us to explore sources for efficient charge separation and enhanced carrier lifetimes important to photocatalytic processes. We find that visible light H2 generation efficiencies in the produced hybrid 1D structures increase in the order CdSe < CdSe/Au NP < CdSe/CdS/Au NP < CdSe/CdS with a maximum H2 generation rate of 58.06 ± 3.59 μmol h(-1) g(-1) for CdSe/CdS core/shell NWs. This is 30 times larger than the activity of bare CdSe NWs. Using femtosecond transient differential absorption spectroscopy, we subsequently provide mechanistic insight into the role nanoscale heterojunctions play by directly monitoring charge flow and accumulation in these hybrid systems. In turn, we explain the observed trend in H2 generation rates with an important outcome being direct evidence for heterojunction-influenced charge transfer enhancements of relevant chemical reduction processes.

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The rapid development of modern industry has exacerbated water pollution, posing significant threats to environmental sustainability and human health. Semiconductor heterostructures have emerged as a promising strategy to enhance the photochemical properties and catalytic activity of heterogeneous catalysts, offering a viable solution to water pollution challenges. In this study, a series of CeO₂/CdSe catalysts was successfully synthesized via chemical deposition. The results demonstrate that CdSe nanoparticles could effectively modulate the band gap, enhance visible light absorption ability, and improve the photochemical performance of CeO₂/CdSe composites. The catalytic activity and practical application prospects of the as-synthesized samples were evaluated by reducing hexavalent chromium (Cr(VI)) and decomposing tetracycline hydrochloride (TCH). CeO₂/CdSe composites exhibited significantly higher photocatalytic activities compared to pure CdSe and CeO₂. Notably, CeO₂/0.6CdSe demonstrated the highest photocatalytic activity, achieving 93.6% removal of Cr(VI), which was 4.39 and 1.77 times as high as those of pure CeO₂ and CdSe in sequence. Furthermore, CeO₂/0.6CdSe achieved a remarkable TCH removal rate of 97.0%. The enhanced photocatalytic performance of CeO₂/0.6CdSe is attributed to the formation of heterojunction structures between CdSe and CeO₂, which facilitates efficient charge separation and transfer. Cyclic experiments confirmed the excellent anti-interference ability and sustainability of CeO₂/0.6CdSe. Reduction mechanisms of Cr(VI) and degradation mechanisms of TCH were thoroughly investigated, with DFT mechanistic analysis providing further insights into the superior catalytic activity of CeO₂/0.6CdSe. This work highlights the potential of CeO₂/0.6CdSe as a highly efficient photocatalyst with broad applications in wastewater treatment.

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Developing type-II heterostructures with a spatial separation of photoexcited electrons and holes is a useful route to promote photocatalytic hydrogen generation. However, few investigations on the charge transfer process across the heterojunction have been carried out, which can allow us to uncover the reaction mechanism. Herein, CdSe quantum dots (QDs) and TiO₂ nanocrystals were synthesized and combined in water yielding CdSe/TiO₂ type II heterostructures. It was found that mercaptopropionic acid as bifunctional molecules could bind with CdSe and TiO₂ to form a cross-linked morphology. The charge carrier dynamics of bare CdSe and CdSe/TiO₂ were detected using femtosecond transient absorption spectroscopy. In the presence of TiO₂, the average exciton lifetime of CdSe QDs was apparently decreased, owing to the electron transfer from photoexcited CdSe to TiO₂. Particularly, the electron-transfer rate from small CdSe QDs (3.0 nm) was much faster than that from big CdSe QDs (4.2 nm). The improved photocatalytic hydrogen generation was observed for CdSe/TiO₂ compared to bare CdSe QDs. The enhancement factor for small CdSe QDs was higher than that for big CdSe QDs, which was in good agreement with the electron-transfer rates. This result indicated that the electron transfer between CdSe and TiO₂ played an important role in photocatalytic hydrogen generation on CdSe/TiO₂ type-II heterostructure. Our study provides a fundamental guidance to construct efficient heterostructured photocatalysts by delicate control of the band alignment.

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Periodic fluorine-doped tin oxide inverse opals (FTO IOs) grafted with CdS nanorods (NRs) and CdSe clusters are reported for improved photoelectrochemical (PEC) performance. This hierarchical photoanode is fabricated by a combination of dip-coating, hydrothermal reaction, and chemical bath deposition. The growth of 1D CdS NRs on the periodic walls of 3D FTO IOs forms a unique 3D/1D hierarchical structure, providing a sizeable specific surface area for the loading of CdSe clusters. Significantly, the periodic FTO IOs enable uniform light scattering while the abundant surrounded CdS NRs induce additional random light scattering, combining to give multiple light scattering within the complete hierarchical structure, significantly improving light-harvesting of CdS NRs and CdSe clusters. The high electron collection ability of FTO IOs and the CdS/CdSe heterojunction formation also contribute to the enhanced charge transport and separation. Due to the incorporation of these enhancement strategies in one hierarchical structure, FTO IOs/CdS NRs/CdSe clusters present an improved PEC performance. The photocurrent density of FTO IOs/CdS NRs/CdSe clusters at 1.23 V versus reversible hydrogen electrode reaches 9.2 mA cm^-2 , which is 1.43 times greater than that of CdS NRs/CdSe clusters and 3.83 times of CdS NRs.

SnSe nanomaterials are challenging to use in sustainable energy production due to difficulties in phase-pure synthesis and efficient charge-carrier separation. We demonstrate a systematic facile synthesis method with an in-depth nucleation and growth mechanism for the rational design of phase-pure and morphology-controlled SnSe-based efficient and cost-effective photocatalysts. Transient absorption spectroscopy measurements are performed to investigate the charge-carrier kinetics of SnSe microflowers (MFs), which exhibit a free charge-carrier lifetime of 6.2 ps. Although the bare SnSe, CdSe, and ZnSe photoanodes demonstrate sizable photocurrents, the construction of CdSe/SnSe and ZnSe/SnSe heterojunctions dramatically improves the photoelectrochemical devices activity. The CdSe/SnSe photoanode shows higher photocurrents of 35 μA cm–2, compared to the ZnSe/SnSe (15 μA cm–2) heterojunction and the individual SnSe (10 μA cm–2), CdSe (7 μA cm–2), and ZnSe (1 μA cm–2). The decent photoactivity of the CdSe/SnSe photoanode is attributed to the desired type-II band alignment and very small band offset (0.08 eV) that exists across the interface, which promotes the efficient separation of photogenerated electron–hole pairs confirmed by cyclic voltammetry measurements and is corroborated by first-principles density functional theory calculations. These findings should open new avenues for the design and development of advanced next-generation tin selenide-based heterostructures for efficient PEC water-splitting applications.

Advanced materials for electrocatalytic and photoelectrochemical water splitting are key for taking advantage of renewable energy. In this study, ZnO/ZnSe/CdSe/Cu(x)S core-shell nanowire arrays with a nanoporous surface were fabricated via ion exchange and successive ionic layer adsorption and reaction (SILAR) processes. The ZnO/ZnSe/CdSe/Cu(x)S sample displays a high photocurrent density of 12.0 mA cm(-2) under AM 1.5G illumination, achieves the highest IPCE value of 89.5% at 500 nm at a bias potential of 0.2 V versus Ag/AgCl, and exhibits greatly improved photostability. The functions of the ZnSe, CdSe, and Cu(x)S layers in the ZnO/ZnSe/CdSe/Cu(x)S heterostructure were clarified. ZnSe is used as a passivation layer to reduce the trapping and recombination of charge carriers at the interfaces of the semiconductors. CdSe functions as a highly efficient visible light absorber and builds heterojunctions with the other components to improve the separation and transportation of the photoinduced electrons and holes. Cu(x)S serves as a passivation layer and an effective p-type hole mediator, which passivates the defects and surface states of the semiconductors and forms p-n junctions with CdSe to promote the hole transportation at the semiconductor-electrolyte interface. The nanoporous surface of the ZnO/ZnSe/CdSe/Cu(x)S core-shell nanowire arrays, together with the tunnel transportation of the charge carriers in the thin films of ZnSe and CdSe, also facilitates the kinetics of photoelectrochemical reactions and improves the optical absorption as well.

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The thermodynamically controlled synthesis of dendritic fractals and nanorods via the hydrothermal reaction has been described, and their extensive photocatalytic hydrogen production properties under simulated solar light have been demonstrated. The long-range and short-range growth of CdSe monomers has been controlled by varying the reaction temperature from 100 to 200 °C. Changes in the physical and optical properties of prepared dendrites and nanorods have been evidently proven with microscopic analysis, diffuse reflectance spectroscopy, and BET analysis. A high-surface area CdSe dendritic fractal has been incorporated with bifunctional Cu3P nanoparticles that resulted in a highly efficient photocatalyst construction. Consequently, a pivotal upswing in the photocatalytic performance of CdSe was found by the formation of the S-scheme heterojunction with Cu3P. The unique properties of transition-metal phosphides kept them as a highly capable co-catalyst to replace precious metals. The physicochemical properties of the prepared materials were characterized by X-ray diffraction, transmission electron microscopy, and X-ray photoelectron spectroscopy. The key challenge in the photocatalytic water splitting process is to develop an efficient photocatalyst not only with high chemical and photochemical stability but also with strong solar light absorption and effective charge separation ability. The co-catalyst Cu3P gives an effective path as it forms the S-scheme heterojunction with CdSe dendritic fractals. This enhances photoactivity and stability of the prepared composite. The composite made of CdSe and Cu3P showed a better rate of H2 production (92.1 mmol h–1 gcat–1) with 4% visible light to hydrogen conversion efficacy. The effects of Cu3P growth, size, and morphology of CdSe on the photocatalytic performance have been studied. Based on the material characterization and photocatalytic activity results, the working mechanism is also proposed.

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The successive ionic layer adsorption and reaction (SILAR) method was used to deposit several CdSe quantum dots (QDs) on the surface of g-C3N4 nanosheets. In comparison to the single moiety of g-C3N4, as-prepared heterostructures displayed an improved bifunctional photo- and electrocatalytic activity for oxygen (OER) and hydrogen evolution reactions (HER). Significantly, the 30 SILAR cycles optimized CdSe QDs/g-C3N4 heterostructure exhibited high performances and stabilities for the OER and HER reaction in alkaline conditions. The as-prepared heterostructure catalyst also exhibited an efficient photocatalytic activity toward the H2 evolution reaction and produced 4306 μmol of H2 gas with 23.8% of apparent quantum yield in the presence of triethanolamine as a sacrificial agent. Photoluminescence spectroscopy, electron paramagnetic resonance, and impedance spectroscopy suggest that the synergy between g-C3N4 nanosheets and CdSe QDs leads to higher catalytic activities, as indicated by the low overpotentials of 147 and 218 mV to obtain a 10 mA cm–2 current density for the HER and OER reactions, respectively. Furthermore, in situ Fourier transform infrared spectroscopy, liquid chromatography–mass spectroscopy, and high-performance liquid chromatography were conducted to determine the photochemical intermediate products to confirm the successful oxidation of TEOA by capturing holes. The outcome is in accordance with the fact that the photogenerated electrons are transferred from the conduction band (CB) of g-C3N4 nanosheets to the valence band (VB) of CdSe QDs in a Z-scheme manner.

In this study, we designed and synthesized photocatalysts for hydrogen evolution from water by coating a thin layer of amorphous TiO2 (a-TiO2) on CdSe nanocrystals (NCs). The thin shell of a-TiO2 serves as a channel for charge carriers otherwise unutilized. Albeit a previous notion that a-TiO2 is a poor photocatalyst, the enhanced photocatalytic activity in the presence of a-TiO2 suggests that the material helps utilize the photogenerated charge carriers when it is in a form of thin shell on CdSe NCs. Type II band offset in CdSe/a-TiO2 appears to allow the electron in the conduction band of CdSe NCs to migrate over to that of a-TiO2, and the electron participates in the hydrogen production from water. Size of CdSe NCs influences the photocatalytic hydrogen evolution rate as the energy difference between the conduction bands of semiconductors becomes larger. Electron transfer from CdSe NCs to a-TiO2 layer is influenced by the level of the conduction-band edge of CdSe NCs: the size dependence indicates that electron injection to TiO2 is facilitated with energy level offset between CdSe and TiO2, while smaller NCs have larger band gap and thus narrower spectral range of absorption. The interplay between charge-transfer rate and absorption cross-section should be considered in designing heterostructure NC-based photocatalysts for water splitting.

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A generic modular synthetic strategy for the fabrication of a series of binary-ternary group II-VI and group I-III-VI coupled semiconductor nano-heterostructures is reported. Using Ag2 Se nanocrystals first as a catalyst and then as sacrificial seeds, four dual semiconductor heterostructures were designed with similar shapes: CdSe-AgInSe2 , CdSe-AgGaSe2 , ZnSe-AgInSe2 , and ZnSe-AgGaSe2 . Among these, dispersive type-II heterostructures are further explored for photocatalytic hydrogen evolution from water and these are observed to be superior catalysts than the binary or ternary semi-conductors. Details of the chemistry of this modular synthesis have been studied and the photophysical processes involved in catalysis are investigated.

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Abstract Solar‐driven photoelectrochemical (PEC) reactions using colloidal quantum dots (QDs) as photoabsorbers have shown great potential for the production of clean fuels. However, the low H2 evolution rate, consistent with low values of photocurrent density, and their limited operational stability are still the main obstacles. To address these challenges, the heterostructure engineering of asymmetric capsule‐shaped CdSe/CdxZn1‐xSe QDs with broad absorption and efficient charge extraction compared to pure‐shell QDs is reported. By engineering the shell composition from pure ZnSe shells into CdxZn1‐xSe gradient shells, the electron transfer rate increased from 4.0 × 107 s −1 to 32.7 × 107 s −1 . Moreover, the capsule‐shaped architecture enables more efficient spatial carrier separation, yielding a saturated current density of average of 25.4 mA cm −2 under AM 1.5 G one sun illumination. This value is the highest ever observed for QDs‐based devices and comparable to the best‐known Si‐based devices, perovskite‐based devices, and metal oxide‐based devices. Furthermore, PEC devices based on heterostructured QDs maintained 96% of the initial current density after 2 h and 82% after 10 h under continuous illumination, respectively. The results represent a breakthrough in hydrogen production using heterostructured asymmetric QDs.

Semiconductors with appropriate band-gaps, band-edges, and lower lattice misfit strain have been assembled to form type II heterostructures to promote the electron–hole separation for photoelectrochemical water splitting. For efficient design of type-II coherent heterostructures, it is essential to calibrate the band gaps, band edge positions, bonding, and lattice misfit strain at the interface. To this end, we apply the first principle study to screen type-II heterostructures by exploring “native” and “non-native” structures of widely used semiconductor materials such as TiO2, ZnO, BiVO4, CdSe, and ZnS. “Non-native” structures differ from the “native” (ground state) structure of bulk crystals in terms of discrete translational symmetry. Due to a change in discrete translational symmetry, atomic, and electronic properties of native and non-native materials are expected to be different. The screening process is based on three criteria: band edges, type-II band alignment between two semiconductors, and identification of coherent interfaces between them. Band edge calculations show that most of the polymorphs are not only suitable for oxygen evolution reaction, but also for the hydrogen evolution reaction. On the basis of band edge positions, several isomaterial and heteromaterial type-II combinations of heterostructures have been explored in which 221 (out of 297) materials have type-II combinations, including many combinations that have not yet been explored experimentally. However, a requirement of the complementary translation symmetry and motif positions to minimize lattice mismatch brings down the possibilities to 19 misfit strained coherent interfaces (e.g., WZ (101̅0)-ZnS/ZB (110)-ZnO). This study points to the centrality of constraints of the coherency of the interface in determining the efficiency of electron–hole separation.

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Heterostructured materials are emerging to improve the performance of hydrogen evolution reaction (HER) catalysts by controlling the interfacial charge-transfer process. Here, we report the synthesis of two-dimensional (2D) CdSe/CdS core/shell nanoplatelets (NPLs) with a 2D MoS2 nanosheet heterostructured electrocatalyst for hydrogen generation. The phonon vibration modes E12g and A1g of pristine MoS2 are upshifted in the case of the heterostructure due to the structural changes induced by stacking or interfacial van der Waals interactions. All of the characteristic peaks of Mo 3d are shifted to lower binding energy in the heterostructure due to the accumulation of electron density around the Mo atom. A promising electrocatalytic activity with an onset overpotential of 171 mV and a Tafel slope of 122 mV dec–1 has been obtained in 2D NPLs-based heterostructured electrocatalysts in acidic medium. Theoretical calculations reveal the creation of a sulfur defect during the HER on heterostructures, leading to excellent activity due to the optimum adsorption of the H* intermediate in the hydrogen-evolving reaction.

1D/2D heterostructures, in particular those that consist of a 1D nanorod core and a 2D nanoplate (NPL) shell, enable the combination of the merits and mitigation of the demerits of distinct dimensionalities into one system, providing a new platform to study their intriguing properties. However, there is still lack of effective strategies to rationally integrate the components with different dimensionalities together. Here, we report a general seeded growth method for the construction of epitaxial 1D/2D heterostructures with a variety of compositional combinations, in which ordered 2D NPL arrays are vertically grown along the c-axis of 1D wurtzite nanomaterials, including II–VI and I–III–VI2 semiconductors. The loading densities of NPLs on the 1D nanomaterials are very high, up to 280 piece/μm. The same crystal structure of the grown NPLs and 1D seeds ensures the epitaxial growth relationship between these two materials. It is found that the secondary 2D growth mode is a kinetic-dominated process, in addition to the effect of the anionic sulfur precursor. The as-prepared 1D/2D CdSe/CdS heterostructures exhibit enhanced activity for photocatalytic hydrogen evolution compared to that of the single-component CdSe NRs and CdS/CdS homostructures. This work greatly enriches the variety and architecture of the available heterostructures and also provides a toolbox for exploring their promising applications.

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Photocatalytic CO2 reduction has been known to be one of the most potential technologies for sustaining the development of human society. However, rapid recombination of photoexcited charges in semiconductors often gets in the way of photocatalytic reaction and annoyingly suppresses the photocatalytic performance. Here, amine-modified step-scheme (S-scheme) porous graphite carbon nitride (g-C3N4)/CdSe–diethylenetriamine (A-PCN/CdSe–DETA) was fabricated via a simple one-step microwave hydrothermal method. The presence of amine functional groups allows for the uniform dispersion of CdSe–DETA on PCN by acting as an organic linker to anchor CdSe–DETA onto PCN nanosheets and forming a close-contact interface. The successful formation of the S-scheme heterojunction along with the built-in electric field between PCN and CdSe–DETA was substantiated through X-ray photoelectron spectroscopy analysis, radical trapping test, and the density functional theory calculation. Taken together, the modification by amine and formation of the S-scheme heterojunction resulted in the optimized A-PCN/CdSe–DETA composite exhibiting extraordinary photocatalytic CO2 reduction performance without the use of a sacrificial agent, achieving a CO production rate of 25.87 μmol/(h g) under visible-light irradiation. This work provides insight into the functionalization of S-scheme photocatalysts using amine functional groups, providing enormous opportunities for various applications beyond photocatalysis.

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We present a combined experimental and theoretical approach for the determination of the low-temperature valence band offset (VBO) at CdSe/ZnTe heterojunctions with underlying zincblende crystal structure. On the experimental side, the optical transition of the type II interface allows for a precise measurement of the type II band gap. We show how the excitation-power dependent shift of this photoluminescence (PL) signal can be used for any type II system for a precise determination of the VBO. On the theoretical side, we use a refined empirical tight-binding parametrization in order to accurately reproduce the band structure and density of states around the band gap region of cubic CdSe and ZnTe and then calculate the branch point energy (also known as charge neutrality level) for both materials. Because of the cubic crystal structure and the small lattice mismatch across the interface, the VBO for the material system under consideration can then be obtained from a charge neutrality condition, in good agreement with the PL measurements.

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We present here the spectroelectrochemical characterization of band edge energetics in Type I versus quasi-Type II core/shell CdSe@CdS nanorod (NR) ensembles, tethered via biphosphonic acids to indium–tin oxide (ITO) electrodes. These investigations targeted either 2.2 or 3.5 nm diameter CdSe seeds embedded in CdS rods with diameters of 5.7 and 7.3 nm and rod lengths of 33 and 37 nm, respectively (CdSe2.2@CdS33 and CdSe3.5@CdS37). CdS shell thicknesses around the CdSe seeds in these NRs were estimated to be ca. 1.7–1.9 nm. Following UV-ozone activation of the NR films, the bleaching of the lowest energy excitonic absorbance features of both the CdSe seeds and the CdS rods was independently monitored as a function of applied (negative) potential. The midpoint potential for bleaching of these excitonic features, associated with electron injection into either the rod or the seed, corrected to a common vacuum scale, provided for estimates of the conduction band edge (ECB) for both the CdSe seed and the CdS rod, with good energy resolution. ECB values in CdSe2.2@CdS35 NRs were found to be the same (ca. −3.99 eV) for both the CdS rod and the CdSe seed, suggesting the formation of a “quasi-Type II” CdS/CdSe heterojunction for CdSe seeds of this diameter, with no detectable energetic offset in ECB. ECB values in the larger seed CdSe3.5@CdS35 NRs were resolved (ca. −4.03 eV for the CdS rods and ca. −3.95 eV for the CdSe seeds), consistent with the formation of a Type I CdS/CdSe heterojunction. These spectroelectrochemical experiments suggest that electrons can be directly injected into both the buried CdSe seed and the CdS rod for Type I heterostructures. Such spectroelectrochemical characterizations provide a direct pathway for estimation of conduction band energies in a variety of complex heterostructured semiconductor nanomaterials.

We report on the fabrication of CdSe quantum dot (QD) sensitized electrodes by direct adsorption of colloidal QDs on mesoporous TiO2 followed by 3-mercaptopropionic acid (MPA) ligand exchange. High efficiency photoelectrochemical hydrogen generation is demonstrated by means of these electrodes. The deposition of ZnS on TiO2/CdSe further improves the external quantum efficiency from 63% to 85% at 440 nm under -0.5 V vs. SCE. Using the same photoelectrodes, solar cells with the internal quantum efficiency approaching 100% are fabricated. The ZnS deposition increases the photocurrent and chemical stability of the electrodes. Investigation of the carrier dynamics of the solar cells shows that ZnS enhances the exciton separation rate in CdSe nanocrystals, which we ascribe to the formation of a type II heterojunction between ZnS and CdSe QDs. This finding is confirmed by the dynamics of the CdSe photoluminescence, which in the presence of ZnS becomes noticeably faster.

We report on noble metal tipping of heterostructured nanocrystals (NCs) of CdSe@CdS tetrapods (TPs) as a chemical reaction to manifest energetic differences between type I and quasi-type II heterojunctions.

Heterostructures of zero-dimensional/two-dimensional (0D/2D) materials, especially quantum dots (QDs)/nanosheets (NSs), have attracted significant attention for extracting photogenerated electrons and holes. Herein, we report the dissociation of excitons at the heterojunction of CdSe (cadmium selenide) QDs and MoS2 (molybdenum disulfide) nanosheet utilizing steady-state and time-resolved spectroscopic techniques. Quasi type II semiconductor-like band energy alignment of the 0D/2D heterojunction facilitates exciton breaking via hole transfer from the QD to MoS2. Furthermore, we demonstrate the extraction of two holes from doubly excited QDs (created via high-power excitation) following the dissociation of a biexciton at the 0D/2D interface. This work is expected to provide a new approach of exploiting multiple exciton generation in quantum dot-sensitized solar cells by harvesting multiple carriers.

Abstract Functionalization of TiO 2 (P25) with oleic acid‐capped CdSe(core)/CdSeTe(crown) quantum‐well nanoplatelets (NPL) yielded remarkable activity and selectivity toward nitrate formation in photocatalytic NO x oxidation and storage (PHONOS) under both ultraviolet (UV‐A) and visible (VIS) light irradiation. In the NPL/P25 photocatalytic system, photocatalytic active sites responsible for the NO(g) photo‐oxidation and NO 2 formation reside mostly on titania, while the main function of the NPL is associated with the photocatalytic conversion of the generated NO 2 into the adsorbed NO 3 − species, significantly boosting selectivity toward NO x storage. Photocatalytic improvement in NO x oxidation and storage upon NPL functionalization of titania can also be associated with enhanced electron‐hole separation due to a favorable Type‐II heterojunction formation and photo‐induced electron transfer from the CdSeTe crown to the CdSe core of the quantum well system, where the trapped electrons in the CdSe core can later be transferred to titania. Re‐usability of NPL/P25 system was also demonstrated upon prolonged use of the photocatalyst, where NPL/P25 catalyst surpassed P25 benchmark in all tests.

The electrochemical stability of MOFs in aqueous medium is most essential for MOFs based electrocatalysts for hydrogen production via water splitting. Since most MOFs suffer from instability issues in aqueous systems, there is enormous demand for electrochemically stable MOFs catalysts. Herein, we have developed a simple postsynthesis surface modification protocol for La (1,3,5-BTC) (H2O)6 metal-organic frameworks (LaBTC MOFs) using Mercaptopropionic acid (MPA), to attain electrochemical stability in aqueous mediums. The MPA treated LaBTC MOFs exhibited better stability than the bare LaBTC. Further, to facilitate light harvesting properties of LaBTC MOFs, Au nanoparticles (NPs) and CdSe quantum dots (QDs) are functionalized on LaBTC. The sensitization of LaBTC with Au NPs and CdSe QDs enhances the light harvesting properties of LaBTC in the visible region of solar spectrum. Using as a photoanode, the electrode generates the current density of ∼80 mA/cm(2) at 0.8 V (vs Ag/AgCl) during photoelectrochemical water splitting. The heterostructured LaBTC photoanode demonstrates the long-term stability for the period of 10 h. The electrode post-mortem analysis confirms the conversion of CdSe QDs into single crystalline 2D-CdS nanosheets. The present investigation reveals that CdS nanosheets together with SPR Au NPs improve the photoelectrochemical water splitting activity and stability of LaBTC MOFs.

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photoelectrochemical device with a novel hierarchical heterostructure coupled with narrow bandgap semiconductors is demonstrated for efficient hydrogen generation via water splitting. The heterostructures consist of ZnO nanowire branches grown on WOx nanowhisker stems, which offer a large surface area and efficient charge transport path. The assembly of CdSe/CdS narrow bandgap cosensitizers on hierarchical ZnO/WOx nanostructures is shown to enhance light harvesting in the visible light region. The cosensitized ZnO/WOx heterostructures demonstrate efficient light absorption up to a wavelength of 800 nm as well as enhanced photoelectrochemical properties when used as photoanodes. Furthermore, CdSe/CdS cosensitized ZnO/WOx has a type II cascade band structure, resulting in efficient charge transport, which was con firmed by open circuit voltage decay measurements. Our photoelectrochemical system produced a high photocurrent density of 11 mA/cm(2) at -0.5 V (vs SCE) under 1.5 AM irradiation for hydrogen generation.

Global environmental issues, in addition to limited fossil fuel resources, are being addressed by quests in search of efficient visible-light-driven water splitting catalysts for hydrogen production. The photocatalytic water splitting activities of CdX/C₂N (X = S, Se) heterostructures have been investigated here using hybrid density functional theory calculations. The calculated band gaps of CdS/C₂N and CdSe/C₂N heterostructures are 1.48 and 2.12 eV, respectively. These are ideal band gap values that make possible harvesting of more visible light from the solar spectrum, which will result in high solar to energy conversion efficiencies. Charge density difference analysis shows that the charge redistributions mainly occur in the interface regions and that the charges transfer from the C₂N to CdX layers. It is interesting to note that the CdX/C₂N heterostructures possess a type-II band alignment, where the relative band alignment of the C₂N and CdX monolayers promotes a spatial separation of the electrons (that resides in C₂N) and holes (that resides in CdX). Importantly, the band edges of the heterostructures straddle the water redox potential under different pH conditions. This study demonstrates that the CdS/C₂N and CdSe/C₂N heterostructures are suitable materials to split water (from various sources) in different ranges of pH values.

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We report the fabrication of quantum dot-sensitized hierarchical structure and the application of the structure as a photoanode for photoelectrochemical water splitting. The structure is synthesized by hydrothermally growing ZnO nanowires on silicon microwires grown with the vapor-liquid-solid method. Then the hierarchical structure is further sensitized with CdS and CdSe quantum dots and modified with IrOx quantum dots. As a result, the silicon microwires, ZnO nanowires, and the quantum dot/ZnO core/shell structure form a multiple-level hierarchical heterostructure, which is remarkably beneficial for light absorption and charge carrier separation. Our experimental results reveal that the photocurrent density of our multiple-level hierarchical structure achieves a surprising 171 times enhancement compared to that from simple ZnO nanowires on a planar substrate. In addition, the photoanode shows high stability during the water-splitting experiment. These results prove that the quantum dot-sensitized hierarchical structure is an ideal candidate for a photoanode in solar water splitting applications. Importantly, the modular design approach we take to produce the photoanode allows for the integration of future discoveries for further improvement of its performance.

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Photoelectrochemical (PEC) water splitting implementing colloidal quantum dots (QDs) as sensitizers is a promising approach for hydrogen (H2) generation, due to the QD's size-tunable optical properties. However, the challenge of long-term stability of the QDs is still unresolved. Here, we introduce a highly stable QD-based PEC device for H2 generation using a photoanode based on a SnO2-TiO2 heterostructure, sensitized by CdSe/CdS core/thick-shell "giant" QDs. This hybrid photoanode architecture leads to an appreciable saturated photocurrent density of ∼4.7 mA cm-2, retaining an unprecedented ∼96% of its initial current density after two hours, and sustaining ∼93% after five hours of continuous irradiation under an AM 1.5G (100 mW cm-2) simulated solar spectrum. Transient photoluminescence (PL) measurements demonstrate that the heterostructured SnO2-TiO2 photoanode exhibits faster electron transfer compared with the bare TiO2 photoanode. The lower electron transfer rate in the TiO2 photoanode can be attributed to slow electron kinetics in the ultraviolet regime, revealed by ultrafast transient absorption spectroscopy. Graphene microplatelets were further introduced into the heterostructured photoanode, which boosted the photocurrent density to ∼5.6 mA cm-2. Our results demonstrate that the SnO2-TiO2 heterostructured photoanode holds significant potential for developing highly stable PEC cells.

Introducing a ternary interlayer into binary heterostructures to construct a ladder band structure provides a promising way for photoelectrochemical water splitting. Here, we design and fabricate a sandwich structure on TiO2 nanotubes using CdSxSe1–x as the interlayer to obtain a matching band alignment. The photoelectrochemical (PEC) properties of composite photoanodes are optimized by the order of sensitization and elements ratio, wherein the TiO2/CdS/CdS0.5Se0.5/CdSe photoanode shows a significantly enhanced photocurrent of 14.78 mA cm–2 at −0.2 V vs SCE, exhibiting a nearly 15-fold enhancement, over 1 order of magnitude. The quantum efficiency apparently increases to 40% at a range of 400–520 nm, resulting from the fact that a sensitizing layer with a matching band alignment can facilitate the separation of photogenerated electron–hole pairs and also extend the absorption range to the visible region due to its narrow bandgaps. Furthermore, its stability was distinctly improved by coating MoS2 on the surface of the TiO2/CdS/CdS0.5Se0.5/CdSe photoanode. Our findings provide a novel route toward developing a highly efficient photoelectrode for water splitting.

We report the fabrication of tuned band gap quantum dots sensitized LaB₆ hybrid nanostructures and their application as a photoanode for photoelectrochemical water splitting. The lanthanum hexaboride (LaB₆) obtained by molten salt electrolysis method is sensitized with different sized CdSe quantum dots, which form a multiple-level hierarchical heterostructure and such design enhance the light absorption and charge carrier separation, which in turn showed higher photocurrent density compared to that of pristine LaB₆. When LaB₆ is sensitized with CdSe quantum dots of different band gaps, which have the absorption in the green and red (530 and 605 nm) regions in visible light, developed a ten times higher photocurrent density (11.0 mA cm(−2)) compared to that of pristine LaB6 (0.5 mA cm(−2) at 0.75 V vs. Ag/AgCl) in 1 M Na₂S electrolyte under illumination. These results prove that the tuned band gap quantum dots sensitized LaB₆ heterostructures are an ideal candidate for a photoanode in solar water splitting applications.

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The efficient conversion of solar energy to chemical energy represents a critical bottleneck to the energy transition. Photocatalytic splitting of water to generate solar fuels is a promising solution. Semiconductor quantum dots (QDs) are prime candidates for light-harvesting components of photocatalytic heterostructures, given their size-dependent photophysical properties and band-edge energies. A promising series of heterostructured photocatalysts interface QDs with transition-metal oxides which embed midgap electronic states derived from the stereochemically active electron lone pairs of p-block cations. Here, we examine the thermodynamic driving forces and dynamics of charge separation in Sb2VO5/CdSe QD heterostructures, wherein a high density of Sb 5s2-derived midgap states are prospective acceptors for photogenerated holes. Hard-x-ray valence band photoemission spectroscopy measurements of Sb2VO5/CdSe QD heterostructures were used to deduce thermodynamic driving forces for charge separation. Interfacial charge transfer dynamics in the heterostructures were examined as a function of the mode of interfacial connectivity, contrasting heterostructures with direct interfaces assembled by successive ion layer adsorption and reaction (SILAR) and interfaces comprising molecular bridges assembled by linker-assisted assembly (LAA). Transient absorption spectroscopy measurements indicate ultrafast (<2 ps) electron and hole transfer in SILAR-derived heterostructures, whereas LAA-derived heterostructures show orders of magnitude differentials in the kinetics of hole (<100 ps) and electron (∼1 ns) transfer. The interface-modulated kinetic differentials in electron and hole transfer rates underpin the more effective charge separation, reduced charge recombination, and greater photocatalytic efficiency observed for the LAA-derived Sb2VO5/CdSe QD heterostructures.