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Here, a facile approach to enhance the performance of solar‐driven photoelectrochemical (PEC) water splitting is described by means of the synergistic effects of a hybrid network of plasmonic Au nanoparticles (NPs) decorated on multiwalled carbon nanotubes (CNTs). The device based on TiO2–Au:CNTs hybrid network sensitized with colloidal CdSe/(CdSexS1−x)5/(CdS)1 core/alloyed shell quantum dots (QDs) yields a saturated photocurrent density of 16.10 ± 0.10 mA cm−2 [at 1.0 V vs reversible hydrogen electrode (RHE)] under 1 sun illumination (AM 1.5G, 100 mW cm−2), which is ≈26% higher than the control device. The in‐depth mechanism behind this significant improvement is revealed through a combined experimental and theoretical analysis for QDs/TiO2–Au:CNTs hybrid network and demonstrates the multifaceted impact of plasmonic Au NPs and CNTs: i) hot‐electron injection from Au NPs into CNTs and TiO2; ii) near‐field enhancement of the QDs absorption and carrier generation/separation processes by the plasmonic Au NPs; iii) enhanced photoinjected electron transport due to the highly directional pathways offered by CNTs. These results provide fundamental insights on the properties of QDs/TiO2–Au:CNTs hybrid network, and highlights the possibility to improve the performance of other solar technologies. [ABSTRACT FROM AUTHOR]

Modulating the heterostructured interface of semiconductor nanocrystals is being widely explored to enhance the charge transfer rate in photoelectrochemical cells. Here we use electrophoretic deposition and a linker-assisted method to fabricate heterostructured photoanodes based on CdSe quantum dots (QDs) or CdSe(S) nanoplates (NPLs)/TiO2 nanoparticles. The semiconductor nanocrystals exhibit red-shifted or dual photoluminescence emissions after electrophoretic deposition, due to the formation of surface traps. The calculated electron transfer rates in CdSe QDs/TiO2 nanoparticle photoanodes fabricated via electrophoretic deposition and 3-mercaptopropionic acid (MPA)-assisted attachment are 5.5 × 107 s−1 and 2.7 × 107 s−1, respectively, which are both higher than those obtained from NPLs/TiO2 nanoparticles ranging from 0.7 × 107 s−1 to 1.3 × 107 s−1. As a proof of concept, we used heterostructured semiconductor nanocrystals/TiO2 nanoparticles as photoanodes for solar-driven hydrogen generation. Upon one sun illumination, the saturated photocurrent densities of photoanodes fabricated via electrophoretic deposition were found to be higher than those of photoanodes prepared via MPA-assisted attachment, while the corresponding photoanode shows inferior long-term photostability due to the formation of surface traps on the QDs or NPLs during the electrophoretic deposition process. These results provide useful information for engineering the surface/dimension/composition of semiconductor nanocrystals, their adsorption on porous TiO2 films, and designing high-efficiency photoelectrodes in photoelectrochemical devices. [ABSTRACT FROM AUTHOR]

Abstract: Colloidal heterostructured quantum dots (QDs) are promising candidates for next‐generation optoelectronic devices. In particular, “giant” core/shell QDs (g‐QDs) can be engineered to exhibit outstanding optical properties and high chemical/photostability for the fabrication of high‐performance optoelectronic devices. Here, the synthesis of heterostructured CuInSexS2−x (CISeS)/CdSeS/CdS g‐QDs with pyramidal shape by using a facile two‐step method is reported. The CdSeS/CdS shell is demonstrated to have a pure zinc blend phase other than typical wurtzite phase. The as‐obtained heterostructured g‐QDs exhibit near‐infrared photoluminescence (PL) emission (≈830 nm) and very long PL lifetime (in the microsecond range). The pyramidal g‐QDs exhibit a quasi‐type II band structure with spatial separation of electron–hole wave function, suggesting an efficient exciton extraction and transport, which is consistent with theoretical calculations. These heterostructured g‐QDs are used as light harvesters to fabricate a photoelectrochemical cell, exhibiting a saturated photocurrent density as high as ≈5.5 mA cm−2 and good stability under 1 sun illumination (AM 1.5 G, 100 mW cm−2). These results are an important step toward using heterostructured pyramidal g‐QDs for prospective applications in solar technologies. [ABSTRACT FROM AUTHOR]

Abstract: “Giant” core/shell quantum dots (g‐QDs) are a promising class of materials for future optoelectronic technologies due to their superior chemical‐ and photostability compared to bare QDs and core/thin shell QDs. However, inadequate light absorption in the visible and near‐infrared (NIR) region and frequent use of toxic heavy metals (e.g., Cd and Pb) are still major challenges for most g‐QDs (e.g., CdSe/CdS) synthesized to date. The synthesis of NIR, heavy metal‐free, Zn‐treated spherical CuInSe2/CuInS2 g‐QDs is reported using the sequential cation exchange method. These g‐QDs exhibit tunable NIR optical absorption and photoluminescence (PL) properties. Transient fluorescence spectroscopy shows prolonged lifetime with increasing shell thickness, indicating the formation of quasi type‐II band alignment, which is further confirmed by simulations. As a proof‐of‐concept, as‐synthesized g‐QDs are used to sensitize TiO2 as a photoanode in a photoelectrochemical (PEC) cell, demonstrating an efficient and stable PEC system. These results pave the way toward synthesizing NIR heavy metal‐free g‐QDs, which are very promising components of future optoelectronic technologies. [ABSTRACT FROM AUTHOR]

Colloidal core/shell quantum dots (QDs) have emerged as a promising light absorber for hydrogen (H 2) production in photoelectrochemical (PEC) cells due to their attractive optoelectronic properties. However, the unfavorable band alignment and lattice mismatch of core/shell materials lead to the sluggish separation/transfer of interfacial charges, challenging the large-scale utilization of QDs-based PEC systems. Herein, we report a unique synthetic approach that incorporates metal ions (Mn2+) into the lattice of heterostructured QDs in the process of shell growth, which is verified by the combined spectroscopic and electrochemical characterizations. The PEC cell based on the optimized QDs (Mn 0.03 -CdSe/CdS) exhibits a superior saturated photocurrent density (∼ 18.7 mA cm−2) to that of undoped CdSe/CdS QDs (∼ 9.6 mA cm−2) under one sun illumination. The introduced intragap states (4T 1) from Mn2+ ions not only reduce the conduction band offset between CdSe and CdS, but also suppress the non-radiative recombination process of photogenerated charges. This work provides a well-grounded guide to utilize metal ions for improving the transfer dynamics of photogenerated charges of heterostructured QDs in other optoelectronic devices. [Display omitted] • Mn2+ dopants are incorporated into the lattice of heterostructured CdSe/CdS QDs in the process of shell growth. • By controlling the concentration of Mn2+, the Mn 0.03 -CdSe/CdS QDs exhibits a superior saturated photocurrent density. • Utilize Mn2+ to improve the transfer rate of photogenerated charges between the core and shell interface. [ABSTRACT FROM AUTHOR]

2D semiconducting nanoplatelets (NPLs) are an emerging class of photoactive materials. They can be used as building blocks in optoelectronic devices thanks to their large absorption coefficient, high carrier mobility, and unique thickness-dependent optical transitions. The main drawback of NPLs is their large lateral size, which results in unfavorable band energy levels and low quantum yield (QY). Here, ultrasmall lead chalcogenide PbSe1- xS x NPLs are prepared, which exhibit an unprecedented QY of ≈60%, the highest ever reported for this structure. The NPLs are applied as light absorber in a photoelectrochemical system, leading to a saturated photocurrent density of ≈5.0 mA cm−2 (44 mL cm−2 d−1), which is a record for NPL-based photoelectrodes in solar-driven hydrogen generation. Ultrasmall NPLs hold the potential for breakthrough developments in the field of optically active nanomaterials. [ABSTRACT FROM AUTHOR]

Solar-driven hydrogen production is one of the most promising strategies for solar-to-hydrogen energy conversion. Compared to inorganic quantum dots (QDs), carbon quantum dots (C-dots) have attracted a lot of attention for optoelectronic devices due to their structure-dependent optical properties and green composition. However, the solar-to-hydrogen conversion efficiency of most of the photoelectrochemical (PEC) devices based on colloidal QDs is still low. Here we demonstrated a highly efficient and stable ecofriendly PEC device using C-dots sensitized TiO 2 photoanode, Pt loaded on carbon nanofibers as counter electrode, and glucose aqueous solution as electrolyte. The red-color C-dots were prepared using a solvothermal reaction, with an absorption spectrum ranging from 300 to 600 nm and a quantum yield (QY) of 50%. The C-dots have excitation independent photoluminescence peak positions and highly crystalline structure. The hydroxyl group on the C-dots can strongly interact with the TiO 2 , forming a very stable complex. Benefiting from these features, the PEC devices based on C-dots exhibit a saturated photocurrent density as high as ~4 mA/cm2 at 0.6 V vs. RHE and the device is very stable (keeping 95% of its initial value after 10-hour illumination upon 100 mW/cm2). This work indicates the promising properties of the C-dots/TiO 2 system, which holds huge potential for applications in the fields of optoelectronic and catalytic devices. [Display omitted] • The red C-dots have an excitation independent emission peak and highly crystalline structure with dominant energy levels. • A PEC device consists of C-dots/TiO 2 as photoanode, Pt/CNFs as CE, and glucose aqueous solution as electrolyte. • The PEC device exhibits saturated photocurrent density as high as ~ 4 mA/cm2 at 0.6 V vs. RHE and the device is very stable. [ABSTRACT FROM AUTHOR]

Colloidal semiconductor quantum dots (QDs) are considered as promising building blocks to fabricate efficient and stable optoelectronic devices, thanks to their size/shape/composition-dependent electronic and optical properties based on the quantum confinement effect. However, inadequate light absorption and undesirable charge recombination events still limit the efficiency and long-term stability of solar energy to fuel conversion in QD-based photoelectrochemical (PEC) cells. In this work, we engineer the optoelectronic properties and band alignment in colloidal heterostructured CdS/CdSe core/shell QDs by tuning the shell thickness. Starting with a 3.0 nm CdS QD core (in diameter), we investigate the changes in structural and optical properties as a function of CdSe shell thickness (0.6–1.9 nm). We show that the optimization of the shell thickness can significantly broaden the light absorption range towards longer wavelengths and enhance the rate of photoelectron separation and transport in photoanodes made of QDs sensitized TiO 2 mesoporous film. Complemented by theoretical modeling, we find that the band alignment in this heterostructured system can be engineered from quasi type-II (electrons are localized over the entire QDs but holes are mostly delocalized in shell) to inverted type-I (both electrons and holes are largely delocalized in shell) by changing the shell thickness. As a proof-of-concept, employing QDs with optimal shell thickness of 1.6 nm together with carbon nanotube doped TiO 2 as photoanode in PEC cells, we obtained a very high photocurrent density of ~ 16.0 mA/cm2 (at 0.9 V vs. the reversible hydrogen electrode, RHE), which is the highest value ever reported for PEC cells based on CdS/CdSe QDs. We also observed an excellent long-term stability (maintaining 83% of its initial value after four hours) under one sun illumination (AM 1.5G, 100 mW/cm2). These results indicate that optimizing the design and band engineering of heterostructured core/shell QDs is a facile and efficient approach to enhance the performance of QDs-based PEC cells and other optoelectronic devices. ga1 • Colloidal core/shell CdS/CdSe QDs were synthesized and their band alignment were engineered by tuning shell thickness. • Optimal PEC device reveals a high photocurrent density of ~ 16.0 mA/cm2 and significantly improved long-term stability. • Theoretical calculation shows that band alignments in this system can be engineered from quasi type-II to inverted type-I. • The improved PEC performance is attributed to more favorable light harvesting and photoelectron transport. [ABSTRACT FROM AUTHOR]

• A facile strategy to align graphene oxide (GO) within 1D cobalt-based nanohybrids (CoNHs). • Addition of GO remarkably enhances the charge transfer within the CoNHs. • GO/CoNHs display outstanding long-term stability (for over 42 h) in alkaline media. • 1D CoNHs (Co 3 O 4) rebuild and regenerate in situ leading to 2D nanosheets (Co 3 S 4 and CoS 2). • GO/CoNHs demonstrate potential as electrocatalysts in solar-driven hydrogen (H 2) generation. Low-cost nanostructured hybrid materials and the optimization of their structural design present an opportunity to achieve high performance and stable renewable energy devices. Here we used electrospinning to homogeneously embed graphene oxide (GO) nanosheets within the one-dimensional structure of cobalt-based nanohybrids (CoNHs). In particular, we focus on the performance of these nanohybrids in Na 2 S/Na 2 SO 3 aqueous electrolyte (pH = 13) due to its wide application in photocatalytic and photoelectrochemical (PEC) devices. We demonstrate that the addition of GO can remarkably reduce the charge transfer resistance from 4.4 Ω to 2.5 Ω for the 0 wt% GO/CoNHs and the 12 wt% GO/CoNHs, respectively. Furthermore, the CoNHs display outstanding electrochemical long-term stability, as the overpotential required to keep J = −10 mA cm−2 is invariable for over 42 h. The structural characterization of the nanohybrids indicates that during continuous operation, the CoNHs rebuild and regenerate in situ leading to the formation of two-dimensional nanostructures comprising a mixture of cobalt chalcogenides (Co 3 S 4 and CoS 2). The integration of the CoNHs in a quantum-dot based PEC cell and an alkaline electrolyzer (1 M KOH) demonstrates the versatility and viability of these alternative electrodes toward active and solar-driven fuel generation. [ABSTRACT FROM AUTHOR]

Photocurrent density of the spherical CdSe/Cd 0.25 Zn 0.75 Se QDs based photoanode, showing a saturated photocurrent density of 13.5 mA/cm2 and a stable H 2 generation under one sun illumination. [Display omitted] • We report a highly stable photoelectrochemical H 2 evolution based on core/shell QDs. • The hole transfer rate is strongly improved by engineering the band structure of QDs. • The device can maintained 99% of its initial current density after 10 h illumination. Colloidal quantum dots (QDs) are promising photosensitizers for the conversion of solar energy into electric energy or chemical fuels. Particularly, they are potentially used in photoelectrochemical (PEC) devices for the realization of solar-driven hydrogen (H 2) generation because their optoelectronic properties could be tunable by controlling their size/shape/composition. However, the long-term operation stability of the PEC devices based on QDs is not good, which is strongly prohibited the potential of the QDs based PEC hydrogen production. Here, we engineer the band structure of the CdSe QDs via a Zn dopant and demonstrate a highly stable PEC devices based on CdSe/Cd 1-x Zn x Se QDs. Through controlling the molar ratio of Cd/Zn precursors, the Zn concentration in the shell can be finely controlled to form an alloyed shell of Cd 1-x Zn x Se. The alloyed shell enables the efficient hole transfer from the QDs to TiO 2 , benefiting from the optimization of the valence energy alignment between the core and shell. As a proof-of-concept, the PEC device based on alloyed-shell QDs achieves a current density of ∼ 13.5 mA/cm2, which is comparable to the best reported PEC devices. More importantly, the PEC devices maintained 99% of its initial current density after 10 h of continuous H 2 generation under one sun illumination. Even after 50 h operation, the PEC device still keeps 65% of initial current density. The excellent long-term operation stability paves the way for the design and use of doped QDs for potential H 2 production. [ABSTRACT FROM AUTHOR]