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Graphene quantum dots (GQDs) commonly suffer from the fluorescence problem of aggregation-caused quenching under high-concentration loading or in the solid state, which seriously hinders the application. Here we report a type of GQDs with red aggregation-induced enhanced emission (AIEE). It is confirmed that the aggregation state of the AIEE GQDs is a J-aggregate. The GQDs/poly(methyl methacrylate) film presented a photoluminescence quantum yield as high as 60.81%, and the record-high performance of luminescent solar concentrators (LSCs) was achieved. The power conversion efficiency (ηPCE) is up to 8.35% and the external optical efficiency (ηext) is ∼8.99% for the GQD-based LSCs (45 mW/cm2). Even under one sun illumination (100 mW/cm2), the corresponding ηPCEand ηextvalues are 3.12% and 4.52%, respectively. The internal photon efficiency (ηint) of an LSC device is about 5.02%. The synthesis of AIEE GQDs bridges the research gap in the emission mechanism of AIEE in GQDs.

InP quantum dots (QDs) are promising building blocks for use in solar technologies because of their low intrinsic toxicity, narrow bandgap, large absorption coefficient, and low-cost solution synthesis. However, the high surface trap density of InP QDs reduces their energy conversion efficiency and degrades their long-term stability. Encapsulating InP QDs into a wider bandgap shell is desirable to eliminate surface traps and improve optoelectronic properties. Here, we report the synthesis of “giant” InP/ZnSe core/shell QDs with tunable ZnSe shell thickness to investigate the effect of the shell thickness on the optoelectronic properties and the photoelectrochemical (PEC) performance for hydrogen generation. The optical results demonstrate that ZnSe shell growth (0.9–2.8 nm) facilitates the delocalization of electrons and holes into the shell region. The ZnSe shell simultaneously acts as a passivation layer to protect the surface of InP QDs and as a spatial tunneling barrier to extract photoexcited electrons and holes. Thus, engineering the ZnSe shell thickness is crucial for the photoexcited electrons and hole transfer dynamics to tune the optoelectronic properties of “giant” InP/ZnSe core/shell QDs. We obtained an outstanding photocurrent density of 6.2 mA cm–1for an optimal ZnSe shell thickness of 1.6 nm, which is 288% higher than the values achieved from bare InP QD-based PEC cells. Understanding the effect of shell thickness on surface passivation and carrier dynamics offers fundamental insights into the suitable design and realization of eco-friendly InP-based “giant” core/shell QDs toward improving device performance.

Fair and meaningful device performance comparison among luminescent solar concentrator-photovoltaic (LSC-PV) reports cannot be realized without a general consensus on reporting standards in LSC-PV research. Therefore, it is imperative to adopt standardized characterization protocols for these emerging types of PV devices that are consistent with other PV devices. This commentary highlights several common limitations in LSC literature and summarizes the best practices moving forward to harmonize with standard PV reporting, considering the greater nuances present with LSC-PV. Based on these practices, a checklist of actionable items is provided to help standardize the characterization/reporting protocols and offer a set of baseline expectations for authors, reviewers, and editors. The general consensus combined with the checklist will ultimately guide LSC-PV research towards reliable and meaningful advances.

Luminescent solar concentrators (LSCs) are able to collect sunlight from a large-area to generate electric power with a low cost, showing great potential in building-integrated photovoltaics. However, the low efficiency of large-area LSCs caused by the reabsorption losses is a critical issue that hampers their practical applications. In this work, we synthesized novel yellow emissive carbon dots (CDs) with a large Stokes shift of 193 nm, which exhibit nearly zero reabsorption. The quantum yield (QY) of the yellow emitting CDs is up to 61%. The yellow emitting CDs can be employed to fabricate high-performance large-area LSCs due to successful suppression of the reabsorption losses. The as-prepared LSCs are able to absorb 14% of the sunlight as the absorption of the CDs matches well with the sun's spectrum. The large-area LSC (10 × 10 cm2) with a laminated structure based on the yellow emitting CDs achieves an optical conversion efficiency (ηopt) of 4.56% and power conversion efficiency (ηPCE) of 4.1% under natural sunlight (45 mW cm−2), which are significantly higher than other previously reported works with similar sizes. Furthermore, the prepared high-performance LSCs show good stability. This method of synthesizing novel CDs for high-efficiency LSCs provides a useful platform for future study and practical application of LSCs.

Carbon quantum dots (C-dots) showed excellent structure-tunable optical properties, mainly composed of carbon, nitrogen and oxygen. They have been used for various types of solid-state optical devices. Due to the photoluminescence quenching caused by aggregation, it is a challenge to produce high quantum yield and large Stokes shift C-dots via controllable and simple approaches. In this work, we demonstrated a microwave assisted heating approach for the high-quality C-dots production with ten grams scale per batch in less than 4 min. The addition of metal cation promoted the formation of the foam-structure by forming carboxyl-metal-amine complex, enabling the spatial confined growth of the C-dots in a solid-state, contributing to the high quantum yield (QY) of 73% with a Stokes shift of 0.65 eV. By tuning the structure of the C-dots, excitation dependent and independent photoluminescent (PL) behavior were achieved because of the formation of the different types of energy states evidenced by transient PL and femtosecond transient absorption spectroscopy. These optical properties enable the C-dots to be successfully integrated in luminescent solar concentrators (LSCs), having an external optical efficiency of 3.0% and a power conversion efficiency of 1.3% (225 cm2) and an excitation-dependent high-level anticounterfeiting fluorescent code, showing a great potential for solid-state optical system.

Despite significant advances in the development of high-efficiency and stable quantum dot (QD) solar cells (QDSCs), recent synthetic and fabrication routes still require improvements to render QDSCs commercially feasible. Here, we describe a low-cost, industrially viable fabrication method of QDSCs under an ambient atmosphere (humid air and room temperature) using stable, high-quality, and small-sized PbS QDs prepared with low-cost, greener precursors [i.e., thioacetamide (TAA)] compared to the widely used bis(trimethylsilyl)sulfide [(TMS)2S], at low temperatures without requiring any stringent conditions. The low reaction temperature, medium reactivity of TAA, and diffusion-controlled particle growth adopted in this approach provide an opportunity to synthesize ultrasmall (emission peak ∼700 nm) to larger PbS QDs (emission peak ∼1050 nm). This also enables well-controlled large-scale (multigram) synthesis with a rough estimated production cost of PbS of 8.11 $ per gram (based on materials cost), which is the lowest among the available PbS QDs produced using wet chemistry routes. QDSCs fabricated using 3.25 nm PbS QDs (bandgap 1.29 eV) under ambient conditions yield a high circuit current density (Jsc) of 32.4 mA/cm2(one of the highest values of Jscever reported) with a power conversion efficiency of 7.8% under 1 sun simulated sunlight at AM 1.5 G (100 mW/cm2). These devices exhibit better photovoltaic performance compared to devices fabricated with more traditional PbS QDs synthesized with (TMS)2S under an ambient atmosphere, confirming the quality of PbS QDs produced with our method. The diffusion-controlled TAA-based synthetic route developed herein is found to be very promising for synthesizing size-tunable PbS QDs for photovoltaic and other optoelectronic applications.

Semiconductor nanocrystals exhibit size-tunable absorption and emission ranging from the ultraviolet (UV) to the near-infrared (NIR) spectral range, high absorption coefficient, and high photoluminescence quantum yield. Effective surface passivation of these so-called quantum dots (QDs) may be achieved by growing a shell of another semiconductor material. The resulting core/shell QDs can be considered as a model system to study and optimize structure/property relations. A special case consists in growing thick shells (1.5 up to few tens of nanometers) to produce “giant” QDs (g-QDs). Tailoring the chemical composition and structure of CdSe/CdS and PbS/CdS g-QDs is a promising approach to widen the spectral separation of absorption and emission spectra (i.e., the Stokes shift), improve the isolation of photogenerated carriers from surface defects and enhance charge carrier lifetime and mobility. However, most stable systems are limited by a thick CdS shell, which strongly absorbs radiation below 500 nm, covering the UV and part of the visible range. Modification of the interfacial region between the core and shell of g-QDs or tuning their doping with narrow band gap semiconductors are effective approaches to circumvent this challenge. In addition, the synthesis of g-QDs composed of environmentally friendly elements (e.g., CuInSe2/CuInS2) represents an alternative to extend their absorption into the NIR range. Additionally, the band gap and band alignment of g-QDs can be engineered by proper selection of the constituents according to their band edge positions and by tuning their stoichiometry during wet chemical synthesis. In most cases, the quasi-type II localization regime of electrons and holes is achieved. In this type of g-QDs, electrons can leak into the shell region, while the holes remain confined within the core region. This electron–hole spatial distribution is advantageous for optoelectronic devices, resulting in efficient electron–hole separation while maintaining good stability.

We report the effect of the integration of carbon dots (Cdots) in high-performance inverted planar-heterojunction (PHJ) perovskite solar cells (PSCs). We used Cdots to modify the hole-transport layer in planar PSC devices. By introducing Cdots on graphene oxide (GO) as hole-transporting layer, the efficiency of the PSC improved significantly from 14.7% in the case of bare GO to 16.2% of the best device with optimized Cdots content. When applying Cdots with an engineered absorption in the UV range as downshifting layer, the device performance was further improved, attaining a maximum PCE of 16.8% (+14%); the stability of the device was also enhanced of more than 20%. Kelvin probe force microscopy (KPFM) and cyclic voltammetry (CV) were employed to analyze the electronic band alignment at the interface between GO/Cdots and the perovskite film. Holes were extracted and transferred to the conductive substrate more efficiently in the presence of Cdots, thus delaying charge recombination. Photoluminescence (PL), transient PL decays and transient photovoltage (TPV) decays investigated the charge-transfer kinetics and proved the retardation of charge recombination. This work reveals an effective enhancement of the performance of planar PSCs by using Cdots/GO as hole transport material.

Luminescent solar concentrators (LSCs) have received significant attention because of their low cost, large-area and high efficiency sunlight energy harvesting. Colloidal core/shell quantum dots (QDs) are promising candidates as absorbers/emitters in LSCs. However, due to the limitation of QDs properties and device architectures, LSCs fabricated using QDs still face the challenges of low optical efficiency and limited long-term stability for the large-area LSCs. In this work, we synthesized CdSe/CdS QDs, and found that higher CdS shell growth temperature results in improved uniformity in structure and morphology and more suitable optical properties. Based on the CdSe/CdS QDs, a large-area (∼100 cm2) sandwich structure luminescent solar concentrator (LSC) was fabricated. By laminating the QDs layer between two sheets of optical clear glass, the reabsorption losses of the device can be reduced due to the decrease of photon escape. The as-fabricated sandwich structure device exhibits an external optical efficiency of ∼ 2.95% under natural sunlight illumination, which represents a 78% enhancement in efficiency over the single layer film LSCs based on CdSe/CdS QDs. More importantly, the sandwich structure can protect the QDs interlayer from the impact of the ambient environment (e.g.oxygen, moisture and alkalinity) and enhance the long-term stability of LSCs. Our work shows that the use of suitably tuned core-shell QDs and the sandwich structure in LSC architecture can dramatically enhance the external optical efficiency of LSC devices based on CdSe/CdS QDs.

Solar-driven photoelectrochemical (PEC) hydrogen (H2) generation is a promising approach to harvest solar energy for the production of a clean chemical fuel. However, the low photon-to-fuel conversion efficiency and long-term stability of PEC devices are major challenges to be addressed to enable large-scale commercialization. Here we report a simple, fast and cost-effective approach to fabricate high efficiency and stable PEC devices for H2generation, by fabricating a hybrid photoanode obtained by incorporating small amounts of multiwall carbon nanotubes (MWCNTs) into a TiO2mesoporous film and sensitizing with colloidal heterostructured CdSe/(CdSexS1−x)5/(CdS)2quantum dots (QDs). The latter were specially designed to accelerate the exciton separation through a band engineering approach. The PEC devices based on the TiO2/QD–MWCNT (T/Q–M) hybrid photoanode with an optimized amount of MWCNTs (0.015 wt%) yield a saturated photocurrent density of 15.90 mA cm−2(at 1.0 VRHE) under one sun illumination (AM 1.5G, 100 mW cm−2), which is 40% higher than that of the reference device based on TiO2/QD (T/Q) photoanodes. This is attributed to a synergistic effect of the promising optoelectronic properties of the colloidal heterostructured QDs and improved electron transport (reduced charge transfer resistance) within the TiO2–MWCNT hybrid anodes enabled by the directional path of MWCNTs for the photo-injected electrons towards FTO. Furthermore, the PEC device based on the T/Q–M hybrid photoanode is more stable (∼19% loss of its initial photocurrent density) when compared with the T/Q photoanode (∼35% loss) after two hours of continuous one sun illumination. Our results provide fundamental insights and a different approach to improve the efficiency and long-term stability of PEC devices and represent an essential step towards the commercialization of this emerging solar-to-fuel conversion technology.

Colloidal quantum dots (QDs) are semiconductor nanocrystals which exhibit discrete energy levels. They are promising building blocks for optoelectronic devices, thanks to their tunable band structure. Here, we explore a nanoengineering approach to highlight the influence of an alloyed interface on the optical and electronic properties of CdSe/(CdS)6“giant” core/shell (CS) QDs by introducing CdSexS1-xinterfacial layers between core and shell. By incorporating of CdSexS1-xinterfacial layers, CdSe/(CdSexS1-x)4/(CdS)2(x = 0.5) core/shell (CSA1) QDs exhibit a broader absorption response towards longer wavelength and higher electron-hole transfer rate due to favorable electronic band alignment with respect to CS QDs, as confirmed by optical absorption, photoluminescence (PL) and transient fluorescence spectroscopic measurements. In addition, simulations of spatial probability distributions show that the interface layer enhances electron-hole spatial overlap. As a result, CSA1 QDs sensitized solar cells (QDSCs) yield a maximum photoconversion efficiency (PCE) of 5.52%, which is 79% higher than QDSCs based on reference CS QDs. To fully demonstrate the structural interface engineering approach, the CdSexS1-xinterfacial layers were further engineered by tailoring the selenium (Se) and sulfur (S) molar ratios during in situ growth of each interfacial layer. This graded alloyed CdSe/(CdSexS1-x)5/(CdS)1(x = 0.9–0.1) core/shell (CSA2) QDs show a further broadening of the absorption spectrum, higher carrier transport rate and modified confinement potential with respect to CSA1 QDs as well as reference CS QDs, yielding a PCE of 7.14%. Our findings define a promising approach to improve the performance of QDSCs and other optoelectronic devices based on CS QDs.

We report a simple, low temperature and solution-processable approach to prepare a composite film of copper sulfide/graphene (CuS-G) as a transparent conducting oxide (TCO) and platinum (Pt)-free CE for Dye-Sensitized Solar Cells (DSSCs). We find that CuS with 3.3 vol% of graphene (CuS-3G) yields the highest power conversion efficiency (PCE) of 4.83%, which is about 12% higher than DSSCs based on CEs made of pristine CuS. After optimizing the graphene concentration, the PCE of the DSSC assembled with the optimized CuS-3G is comparable to that based on Pt CE. The similar performance of the CuS-3G CE compared with Pt CE is mainly attributed to the small series resistance and high electrocatalytic activity of the CuS-3G CE; this is confirmed by cyclic voltammetry and electrochemical impedance spectroscopy. These results indicate a straightforward methodology for the low cost and easy synthesis of an alternative CE in DSSCs.

Colloidal semiconductor quantum dots (QDs) have attracted a great attention for their potential applications in optoelectronic devices, such as water splitting, luminescent solar concentrators, and solar cells, because of their size/shape/composition-dependent optoelectronic properties. However, the fast electron-hole (e-h) recombination and slow charge separation of QDs limit their applications as light absorbers in high-efficiency optoelectronic devices. Here, we synthesized thick-shell CdSe/CdSexS1-x/CdS QDs with pyramidal shape, which exhibit a quantum yield of ~ 15%, with a long radiative lifetime up to ~ 100 ns due to the spatial separation of the e/h wavefunction and significantly broadened light absorption toward the 500–700 nm range, compared to CdSe/CdS unalloyed QDs. As a proof-of-concept, the pyramidal QDs are applied as light absorbers in a photoelectrochemical (PEC) system, leading to a saturated photocurrent density of ~ 12 mA/cm2(with a H2generation rate of 90 mL cm−2day−1), which is a record for thick-shell QD-based photoelectrodes in PEC hydrogen generation. Core/thick-shell QDs hold great potential for breakthrough developments in the field of QD-based optoelectronic devices.

Metal selenides have caused widespread concern due to their high theoretical capacities and appropriate working potential; however, they suffer from large volume variation during cycling and low electrical conductivity, which limit their practical applications. In this article, a three-dimensional (3D) porous carbon framework embedded with homogeneous FeSe2nanoparticles (3D porous FeSe2/C composite) was synthesized by a facile calcined approach, following a selenized method without a template. As the uniformity of FeSe2nanoparticles and 3D porous structure are beneficial to accommodate volume stress upon cycling and shorten electrons/ions transport path, associated with carbon as a buffer matrix for increasing conductivity, the 3D porous FeSe2/C composite displays excellent electrochemical properties with high reversible capacities of 798.4 and 455.0 mA h g–1for lithium-ion batteries and sodium-ion batteries, respectively, when the current density is 100 mA g–1after 100 cycles. In addition, the as-prepared composite exhibits good cycling stability as compared to bare FeSe2nanoparticles. Therefore, the facile synthetic strategy in the current work provides a new perspective in constructing a high-performance anode.

Luminescent solar concentrator (LSC) can serve as large-area sunlight collectors, suitable for applications in building-integrated high-efficiency and low-cost photovoltaics. Inorganic perovskite quantum dots (QDs) are promising candidates as absorbers/emitters in LSCs, due to their high quantum yields (close to 100%), possibility of tuning size and chemical composition and broad absorption spectrum and high absorption coefficient. However, despite their great potential for technological development, LSCs fabricated using colloidal perovskite QDs still face major challenges such as low optical efficiency and limited long-term stability. Here we report a large-area (~ 100 cm2) tandem LSC based on nearly reabsorption-free carbon dots (C-dots) and inorganic mixed-halide perovskite QDs spectrally-tuned for optimal solar-spectrum splitting. The as-fabricated semi-transparent device, without involving any complicated processes, exhibits an external optical efficiency of ~ 3% under sunlight illumination (100 mW/cm2), which represents a 27% enhancement in efficiency over single layer LSCs based on CsPb(BrxI1-x)3QDs and 117% over CsPb(ClxBr1-x)3QDs. Our work shows that the addition of C-dots can dramatically enhance the long-term durability of LSC devices based on perovskite QDs due to their excellent photostability and simultaneous absorption of ultraviolet light.

Luminescent solar concentrators (LSCs) can serve as large-area sunlight collectors for photovoltaic (PV) cells, reducing the cost of PV generated power. Typical LSCs consist of optical waveguides doped with highly emissive fluorophores e.g. quantum dots (QDs) or dyes/polymers which are required to exhibit high optical efficiency and long-term stability. Compared to conventional fluorophores, carbon dots (C-dots) have superior advantages of non-toxicity, environmental friendliness, low-cost and simple preparation using abundant carbon based feedstock. Here, we demonstrate large-area LSCs (up to 100cm2) using colloidal C-dots. Two types of LSCs were fabricated by either incorporating oil-soluble oleylamine-treated C-dots into photo-polymerized poly(lauryl methacrylate) (PLMA) or spin-coating the water-soluble C-dots/polyvinylpyrrolidone (PVP) mixture on the glass substrate. LSCs based on C-dots/PLMA exhibit a quantum efficiency of 4% (geometric (G) factor of 38) and an optical efficiency of 1.1% (100cm2, G factor of 12.5) of tandem thin-film LSCs based on C-dots/PVP was achieved under one sun illumination. This performance is comparable to those of LSCs based on inorganic QDs with similar G factor. The LSCs based on C-dots are highly air-stable without any noticeable variation in photoluminescence under ultraviolet illumination (1.3W/cm2) for over 12h.

Colloidal core/shell quantum dots (QDs) have emerged as a promising light absorber for hydrogen (H2) 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 (Mn0.03-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 (4T1) 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.

Luminescent solar concentrators (LSCs) are considered a promising technology to reduce the cost of electricity by decreasing the use of expensive photovoltaic materials, such as single-crystal silicon. In addition, LSCs are suitable for applications in building-integrated photovoltaics. Inorganic perovskite quantum dots (QDs) are promising candidates as absorbers/emitters in LSCs, due to their excellent optical properties including size/chemical-composition dependent absorption/emission spectrum, high absorption coefficient, high quantum yield and good stability. However, due to the large overlap between their absorption and emission spectra, it is still very challenging to fabricate large-area high-efficiency LSCs using perovskite QDs. Here we report the synthesis of mixed-halide perovskite CsPb(BrxI1−x)3QDs with small overlap of absorption and emission spectra, high quantum yield (over 60%) and absorption spectrum ranging from 300 to 650nm. We use these QDs to build semi-transparent large-area LSCs that exhibit an external optical efficiency of 2% with a geometrical gain factor of 45 (9cm in length). To date, these represent the brightest and most efficient solution-processed perovskite QDs based LSCs compared to LSCs based on perovskite thin films. The LSCs exhibit long term air stability without any noticeable variation in photoluminescence and lifetime under 4 W UV light illumination for over four hours.

Photoelectrochemical (PEC) hydrogen generation based on colloidal quantum dots (QDs) is very promising because of its high solar energy to fuel conversion efficiency and low fabrication costs. However, its commercial development is hindered by various challenges, including the widespread use of toxic heavy metal–based QDs as sensitizers. We report an environmentally friendly, high efficiency PEC device in which the photoanode consists of a mesoporous TiO2film sensitized with heavy metal-free, near-infrared (NIR) colloidal CuInSexS2−x(CISeS) QDs. To reduce surface-related traps, we grew an ultrathin ZnS shell on the CISeS core QDs by cation exchange. The PEC cell based on this core/shell CISeS/ZnS QDs exhibits suppressed charge recombination and a saturated photocurrent density of ~5.3mA/cm2under one sun illumination (AM 1.5G, 100mW/cm2). In addition, the as-prepared PEC device shows an outstanding stability, exhibiting a drop of only 23% after 9h illumination. The success in using such core/shell CISeS/ZnS QDs paves the way to realize environment-friendly, high efficiency and cost-effective PEC devices for hydrogen production.

The interfacial structure in “giant” PbS/CdS quantum dots (QDs) was engineered by modulating the Cd:S molar ratio during in situgrowth. The control of the gradient interfacial layer could facilitate hole transfer, regulate the transition from double- to single-color emission, as a consequence. These QDs are optically active close-to-the near-infrared (NIR) spectral region and are candidates as absorber materials in solar energy conversion. Photoinduced charge transfer from “giant” QDs to electron scavenger can still take place despite the ultra-thick (~5nm) shell. The hybrid architecture based on a TiO2mesoporous framework sensitized by the “giant” QDs with alloyed interface can produce a saturated photocurrent density as high as ~5.3mA/cm2in a photoelectrochemical (PEC) cell under 1 Sun illumination, which is around 2 times higher than that of bare PbS and core/thin-shell PbS/CdS QDs sensitizer. The as-prepared PEC device presented very good stability thanks to the “giant” core/shell QDs architecture with tailored interfacial layer and a further coating of the ZnS shell. 78% of the initial current density is kept after 2-h irradiation at 1 Sun. Engineering of electronic band structure plays a key role in boosting the functional properties of these composite systems, which hold great potential for H2production in PEC devices.

Benefiting from the wide-range absorption and adjustable energy gap, carbon dots (C-dots) have attracted a great deal of attention and they have been used to sensitize semiconductor nanocomposites to boost the efficiency of energy conversion devices, while there is still a lack of fundamental understanding of the interaction between such materials and their influence on the catalytic activity on the reaction process. In this study, C-dots were used to modify TiO2to form a direct Z-scheme (DZS) junction for enhancement of the photocatalytic activity. The C-dot/TiO2composite was prepared by ultrasonication at room temperature through coupling between the Ti–O–C bond and electrostatic interaction. The C-dots can dramatically enhance the absorption of the composite by forming the DZS, and the composite is enabled to generate more free radicals, which facilitate ∼10 times higher photocatalytic activity compared to that of TiO2. As a proof of concept, the as-prepared C-dot/TiO2was used for textile wastewater dye degradation. This study provides an efficient approach for room-temperature preparation of C-dot/TiO2composites with high photocatalytic activity.

Quantum dot (QD) sensitized TiO2is considered as a highly promising photoanode material for photoelectrochemical (PEC) solar hydrogen production. However, due to its limited stability, the photoanode suffers from degradation of its long-term PEC performance. Here, we report the design and characterization of a high-efficiency and long-term stable Pt-free PEC cell. The photoanode is composed of a mesoporous TiO2nanoparticle film sensitized with “giant” core@shell QDs for PEC solar hydrogen generation. The thick shell enhances light absorption in the visible range, increases the stability of the QDs and does not inhibit charge separation, injection and transport, needed for proper operation of the device. We prepared thin films of Cu2S nanoflakes through a simple and reproducible procedure, and used them as counter-electrodes replacing the standard Pt film, resulting in equivalent performances of the PEC cell. We obtained an unprecedented photocurrent density (~10mA/cm2) for “giant” QDs based PEC devices (and corresponding H2generation) and a very promising stability, indicating that the proposed cell architecture is a good candidate for long-term stable QD-based PEC solar hydrogen generation.

Colloidal Quantum dots (QDs) are considered promising light harvesters for photoelectrochemical (PEC) hydrogen generation devices due to their size tunable optoelectronic properties. However, the solar-to-hydrogen (STH) efficiency and long-term stability of devices based on QDs are still relatively low, thus limiting the commercial development. These limitations are attributed to the limited absorption range of QDs, unfavorable band energy alignment and photo-oxidation. Here we propose and realize two core/multiple-shell architecture based on CdSe/CdS/ZnS QDs. The shell composition is optimized with gradient layers, forming CdSe/CdSexS1−x/CdS/ZnyCd1−yS/ZnS core/multiple-shell structures, which reduces the surface traps and defects of QDs and simultaneously suppresses exciton recombination by providing intermediated alloyed interlayers. The PEC device based on a mesoporous TiO2sensitized with two types of core/multiple-shell QDs exhibited an outstanding saturated photocurrent density of 20.5 mA/cm2for alloyed core/multiple-shell QDs, under one sun illumination (AM 1.5 G, 100 mW/cm2). To our knowledge, this is comparable to the highest value reported so far for the PEC devices based on colloidal QDs. In addition, the as-prepared PEC devices exhibited excellent stability, maintaining ~93.4 % of the initial photocurrent density after 2-hour continuous illumination (100 mW/cm2). This work provides an efficient approach for improving the performance of PEC devices through QDs structure engineering.

The effect of PbS core size on the temperature-dependent photoluminescence (PL) of PbS/CdS quantum dots (QDs) in the temperature range of 100–300 K was thoroughly investigated and compared with shell-free PbS QDs. The core/shell QDs show significantly smaller PL intensity variation with temperature at a smaller PbS size, while a larger activation energy when the PbS domain size is relatively large, suggesting both different density and different distribution of defects/traps in the PbS and PbS/CdS QDs. The most remarkable difference consists in the PbS size dependence of the energy gap temperature coefficient (dE/dT). The PbS/CdS QDs show unusual non-monotonic dE/dTvariation, resulting in the reversal of the dE/dTdifference between the PbS and PbS/CdS QDs at a larger PbS size. In combination with theoretical calculations, we find that, although lattice dilation and carrier-phonon coupling are generally considered as dominant terms, the unique negative contribution to dE/dTfrom the core/shell interfacial strain becomes most important in the relatively larger-core PbS@CdS QDs.

The luminescence switching behavior of CePO4:Tb has been widely studied upon an interfacial oxidation–reduction reaction where KMnO4and ascorbic acid act as an oxidant and a reductant, respectively. However, the transformation of Mn-involved species derived from KMnO4during the oxidation–reduction cycle and their effect on the luminescence properties of CePO4:Tb have not been explored so far. Here, we further study this interfacial reaction between CePO4:Tb and KMnO4through various characterization techniques, such as X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. We find that an amorphous manganese oxide layer forms on the CePO4:Tb surface along with the partial oxidation of Ce(III) upon addition of KMnO4. In the subsequent reduction, the ascorbic acid not only reduces Ce(IV) to Ce(III) but also dissolves the formed manganese oxide. If manganese oxide is kept on the CePO4:Tb surface during the reduction treatment, the photoluminescence of Tb(III), due to the energy transfer from Ce(III) to Tb(III), would be restrained even if Ce(IV) ions were efficiently reduced. Although the degree of surface oxidation/reduction (Ce(III)/Ce(IV)) was considered to be a key factor for the luminescence quenching/recovery behavior in previous studies, there is a strong indication that the reaction product, e.g. manganese oxide, and associated surface defects generated from the oxidation–reduction reaction can disturb the photoluminescence of Tb(III) when they are not removed.

Abstract: Optical properties and ultrafast exciton relaxation dynamics in PbS and core/shell PbS/CdS quantum dots (QDs) have been studied using UV-vis absorption and fluorescence spectroscopy as well as femtosecond (fs) transient absorption spectroscopy. The electronic absorption spectrum of the PbS QDs features broad absorption in the entire near IR-vis-UV region with a monotonic increase in intensity towards shorter wavelength. Relative to PbS, the absorption of the core/shell PbS/CdS QDs shows a slight blue shift in the 600–800 nm region, due to the decrease of the PbS crystal size caused by the synthetic process of the core/shell structure, and increased absorption near 400 nm due to the CdS shell. The PL of the PbS/CdS QDs was ∼2.6 times more intense than that of the PbS QDs, due to surface passivation of PbS by CdS, and blue-shifted, attributable to smaller PbS size and thereby stronger quantum confinement in the core/shell QDs. Fs transient absorption measurements of both systems showed a strong transient absorption feature from 600 to 750 nm following excitation at 750 nm. The transient absorption decays can be fit to a biexponential with time constants of 8 and 100 ps for PbS and 6 and 80 ps for PbS/CdS. The amplitude and lifetime of the fast component were excitation intensity dependent, with the amplitude increasing more than linearly with increasing excitation intensity and the lifetime decreasing with increasing intensity. The fast decay is attributed to exciton-exciton annihilation and it occurs more readily for the PbS/CdS than the PbS QDs, which is attributed to a lower density of trap states in the core/shell QDs, as supported by their stronger PL.

A solution‐processed nanoarchitecture based on PbS quantum dots (QDs) and multi‐walled carbon nanotubes (MWCNTs) is synthesized by simply mixing the pre‐synthesized high‐quality PbS QDs and oleylamine (OLA) pre‐functionalized MWCNTs. Pre‐functionalization of MWCNTs with OLA is crucial for the attachment of PbS QDs and the coverage of QDs on the surface of MWCNTs can be tuned by varying the ratio of PbS QDs to MWCNTs. The apparent photoluminescence (steady‐state emission and fluorescence lifetime) “quenching” effect indicates efficient charge transfer from photo‐excited PbS QDs to MWCNTs. The as‐synthesized PbS‐QD/MWCNT nanoarchitecture is further incorporated into a hole‐conducting polymer poly(3‐hexylthiophene)‐(P3HT), forming the P3HT:PbS‐QD/MWCNT nanohybrid, in which the PbS QDs act as a light harvester for absorbing irradiation over a wide wavelength range of the solar spectrum up to near infrared (NIR, ≈1430 nm) range; whereas, the one‐dimensional MWCNTs and P3HT are used to collect and transport photoexcited electrons and holes to the cathode and anode, respectively. Even without performing the often required “ligand exchange” to remove the long‐chained OLA ligands, the built nanohybrid photovoltaic (PV) device exhibits a largely enhanced power conversion efficiency (PCE) of 3.03% as compared to 2.57% for the standard bulk hetero‐junction PV cell made with P3HT and [6,6]‐Phenyl‐C61‐Butyric Acid Methyl Ester (PCBM) mixtures. The improved performance of P3HT:PbS‐QD/MWCNT nanohybrid PV device is attributed to the significantly extended absorption up to NIR by PbS QDs as well as the effectively enhanced charge separation and transportation due to the integrated MWCNTs and P3HT. Our research results suggest that properly integrating QDs, MWCNTs, and polymers into nanohybrid structures is a promising approach for the development of highly efficient PV devices.

Successful transfer of near-infrared emitting quantum dots (QDs) into water is critical for many biological applications. In this paper, PbS QDs capped by several types of most commonly used capping ligands are transferred from an organic solvent into water via poly(maleic anhydride-alt-1-octadecene)-co-poly(ethylene glycol). It is found that the variation of the structure and optical property of QDs upon water transfer highly depends on the type of capping ligands because of their different interactions with their surroundings (such as solvent molecules, polymers, and QD surface). This is in clear contrast to the common concept that amphiphilic polymers will not disturb the ligand structure on the QD surface during the transfer process, and thus the type of ligands will not bear any direct relevance with the variation of QDs and their properties. This work demonstrates that when oleic acid (OA) or OA/trioctylphosphine (TOP) ligands are used, the amphiphilic polymer approach is able to maintain the initial QD structure and high photostability in water; whereas, in the case of oleylamine (OLA) ligands, severe ligand etching takes place, which initiates Ostwald ripening, leading to double size distribution and, moreover, the total photoluminescence (PL) loss in a short time after water transfer. The complete darkening of PbS capped by OLA QDs is found to be mainly due to the introduction of many unpassivated surface atoms during the etching and Ostwald ripening process. Among all the samples, PbS QDs capped by OA/TOP ligands show the highest PL intensity, doubling that of PbS QDs capped by OLA or OA ligands after immediate water transfer. Meanwhile, the PL spectral shift after water transfer also varies among the samples. Our study suggests that the surface ligands do play a crucial role in the process of water transfer of PbS QDs, and “correct” ligands should be used to obtain high-quality water-soluble PbS QDs using the amphiphilic polymer approach.

The concentration-dependent photoinduced photoluminescence (PL) enhancement of PbS quantum dots (QDs) is observed for the first time in the PbS colloidal solution. The enhancement in PL efficiency is attributed to the photooxidation of PbS QD surface based on the optical, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) measurements. The decrease in PbS QD concentration leads to the increase in the amplitude of PL enhancement and to the decrease in the time required to reach the maximum PL enhancement. For the lowest investigated QD concentration of 58.7 nmol/L, PL efficiency can be increased by as much as 60 times. The critical concentration for realizing the considerable PL enhancement is found to be ∼200 nmol/L. Further investigation shows that the concentration-dependent PL enhancement is related to spontaneous ligand desorption and therefore more efficient photooxidation in low-concentration samples. Furthermore, it was found that the enhanced PL can remain during storage. It suggests that the postsynthesis treatment of UV illumination can serve as an alternative, while simple and highly efficient way in improving the PL efficiency of PbS QDs when the coverage of surface ligands is appropriately controlled.