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Fluorescent carbon dots (C-dots) with high optical properties and biocompatibility have great potential for various types of optoelectronic applications. Although many approaches have been used for producing high-quality C-dots, it is still a challenge to produce high-quality C-dots with required properties via a simple and efficient approach. In this work, by controlling the addition of different types of divalent (e.g. Mn and Sr) or trivalent (e.g. Al, Ga and In) metal ions, we synthesized fluorescent C-dots with single or double emissions with quantum yield (QY) as high as 61% using a simple heating approach. Transient fluorescence spectroscopy and transient absorption spectroscopy reveal that the double emission and an excitation-dependent photoluminescence behavior of trivalent ion-capped C-dots originate from the existence of multiple energy levels. Based on these optical properties, the as-prepared C-dots can be effectively used as building blocks for fabricating various types of optoelectronic devices. A single emission Mn-C-dot based large-area luminescent solar concentrator (LSC) (225 cm2) achieved a power conversion efficiency (PCE) of 1.3% under natural sunlight (50 mA cm−2); double emission Al–C-dots showed excitation wavelength dependent fluorescent codes, showing great potential for application in developing anti-counterfeiting codes; and magnetic tests showed that Mn-capped C-dots are superparamagnetic and this may be useful for magnetically driven fluorescence imaging in biomedical applications. This study offers an eco-friendly and efficient approach for producing large-scale high-quality C-dots, demonstrating great potential for optical and electrical applications. [ABSTRACT FROM AUTHOR]

Luminescent solar concentrators (LSCs) are large-area sunlight collectors for efficient solar-to-electricity conversion. The key point for highly efficient LSCs is the choice of fluorophores, which need to have broad absorption, high quantum yield and large Stokes shift. Among various fluorophores, carbon quantum dots (C-dots) hold great promise as eco-friendly alternatives to heavy-metal-containing quantum dots (QDs) due to their adjustable absorption and emission spectra, non-toxicity, low cost and eco-friendly synthetic methods. However, due to the limited absorption band and relatively low quantum yield in the red region, it is a challenge to obtain efficient LSCs based on C-dots. Here, we demonstrated highly efficient LSCs based on red-emissive C-dots. The as-synthesized C-dots have a cubic structure, broad absorption covering 300–600 nm, and red emission (peak located at 595 nm), with a high quantum yield of ∼65% and a large Stokes shift of 0.45 eV. Transient absorption experiments of the C-dots revealed the ultrafast formation of the broad emissive state (1 ps). Based on the excellent optical properties of the C-dots, the as-prepared large-area LSC (10 × 10 × 0.52 cm3) exhibited an optimized external optical efficiency of 4.81% and a power conversion efficiency of 2.41% under natural sun irradiation (70 mW cm−2). Furthermore, a tandem LSC using green-emissive C-dots (top layer) and red-emissive C-dots (bottom layer) as fluorophores exhibited an external optical efficiency as high as 6.78%. These findings demonstrate the possibility of using eco-friendly carbon-based nanomaterials for highly efficient large-area LSCs. [ABSTRACT FROM AUTHOR]

Luminescent solar concentrators (LSCs) are large-area sunlight collectors, and they are able to reduce the cost of electricity and are suitable for application in building integrated photovoltaics. The key factor determining the optical efficiency of the LSC is the optical properties of the fluorophores. Among various types of fluorophores, carbon quantum dots (C-dots) have attracted great attention because of their size/shape/composition dependent optical properties and high stability. Although LSCs based on C-dots have been reported recently, it is still a big challenge to produce highly-bright cost-effective C-dots on a large-scale via a simple approach for efficient LSC fabrication. In this work, we demonstrate a simple solvent-free heating method using metal salts as dehydrating agents to produce C-dots in a large-quantity (100 gram per batch). The as-prepared C-dots have a typical absorption ranging from 300 to 500 nm, a large Stokes shift of 0.73 eV, a quantum yield (QY) as high as 70% and a small absorption/emission spectral overlap. The metal cation can not only contribute to the dehydration of the precursors, but also cap the C-dot surface by forming a stable structure, leading to a strong absorption peak at 405 nm and a large Stokes shift because of the intermediate energy states. Compared to Cu, Fe, Mg, Al, and Sr cations, Ca2+/Ba2+ capped C-dots exhibited the highest QY because of the efficient surface passivation. As a proof-of-concept, we fabricated the laminated LSCs using the as-obtained C-dots as fluorophores. The optimized LSC based on C-dots produced using BaCl2 or CaCl2 exhibits an external optical efficiency as high as 3.2% with a lateral size of 100 cm2. These results indicate that the solvent-free heating method can provide high-quality C-dots as building blocks for potential LSC applications. [ABSTRACT FROM AUTHOR]

Compared to inorganic quantum dots, fluorescent carbon nanomaterials (C-dots) have gained significant attention because of their unique optoelectrical properties and low toxicity. Although many review articles summarized the last research achievements, only a few of them are focusing on red/near-infrared C-dots. Due to their unique optical and optoelectrical properties in the red/near-infrared region, this interesting subclass of C-dots may be applied as important building blocks for several applications spanning from bioimaging and nano-thermometry, to luminescent solar concentrators (LSCs) and photoelectrochemical systems. Therefore, in this review the synthesis and the fluorescence mechanism together with the most important applications in thermometry, bio-imaging, LSCs and photocatalysis of red/near-infrared C-dots are considered. Furthermore, another aim is to highlight the available approaches to improve the carbonization degree and, additionally, to discuss the structure/composition correlated optical properties. Finally, outlooks, future perspectives and challenges are also discussed for these highly promising nanostructures. [ABSTRACT FROM AUTHOR]

Carbon quantum dots (C-dots) have attracted increasing attention because of their nontoxic composition and excellent optical properties, which have seen them widely used as building blocks for various types of optoelectronic devices. However, the large-scale synthesis of C-dots with a high quantum yield is a big challenge. Herein, C-dots were synthesized via a vacuum heating approach, which could provide over 100 g per batch. The purified C-dots had an ultrahigh quantum yield of ∼79%, absorption peaks at 330 and 405 nm, and a narrow fluorescence spectrum with a main peak centered at 520 nm. As a proof of concept, an as-prepared C-dots/polymer aqueous solution was used as a security ink for textile anti-counterfeiting. After printing the ink into the cotton fabrics, the C-dots still had a quantum yield of 42%. By mixing with other colored C-dots, a multiple colored anti-counterfeiting security code was obtained, which could greatly enhance the anti-counterfeiting level. This study provides a reliable approach to provide high-quality C-dots in a large scale and may open possibilities to use C-dots for potential flexible anti-counterfeiting and optoelectronic applications. [ABSTRACT FROM AUTHOR]

Luminescent solar concentrators (LSCs) are large-scale sunlight collectors, consisting of fluorophores embedded in waveguides, which can concentrate part of the absorbed sunlight at the borders of the slab through wave-guided photoluminescence. Benefiting from their low-cost and semi-transparency, they exhibit great potential for building integrated photovoltaics. Among various types of fluorophores, carbon quantum dots (C-dots) have attracted great interest due to their relatively high quantum yield (QY), low-cost, non-toxic composition and simple synthetic methods. Unfortunately, most red-emitting C-dots with high QYs were synthesized using relatively toxic and expensive precursors. The C-dots exhibiting red-emission synthesized using sustainable precursors (e.g. citric acid) have QYs less than 20%. Here we synthesized the red-emitting C-dots produced by using citric acid and urea as precursors and N,N-diethylformamide as the solvent via a solvothermal reaction. The red C-dots have a broad absorption from 300–650 nm, with a QY as high as 40% in ethanol. In addition, the C-dots exhibited good biocompatibility, even for a C-dot concentration up to 1000 μg mL−1. The LSC (LSC area 100 cm2) based on red C-dots exhibited a solar-to-electricity power conversion efficiency (PCE) of 1.9% under natural sunlight illumination (35 mW cm−2). We combined red-emitting C-dots with green-emitting C-dots prepared via a vacuum heating approach. By using a tandem structure, composed of two slabs each incorporating a different C-dot type, the obtained PCE of the LSC based on the combination of red and green C-dots further increases up to 2.3% (under the same irradiance equal to 35 mW cm−2), which is comparable to the reported PCEs for the LSCs based on C-dots or other types of fluorophores. This work indicates that the red-emitting C-dots produced by low-cost and environmentally-friendly precursors exhibit great potential as building blocks for the environmentally compatible LSCs. [ABSTRACT FROM AUTHOR]

Optical nanothermometers have attracted much attention due to their non-contact and precise measurement with high spatial resolution at the micro- and nanoscales. They can be applied in various fields such as micro-opto-electronics, photonics, and biomedical thermal and pH sensing, while most thermal sensors reported so far contain heavy metals or have low sensitivity. Herein, we demonstrate a highly sensitive ratiometric thermal sensor based on colloidal C-dots. C-dots exhibit dual emission originating from the band gap emission and surface-dominant emission, which show a different temperature-dependent photoluminescence (PL) response. Among different surface-functionalized C-dots, C-dots@OH exhibit an absolute thermal sensitivity of −0.082 °C−1, which is the highest among various types of ratiometric thermosensors, making it a very promising candidate for high-sensitivity, self-calibrated nanoscale thermometry. As a proof-of-concept, C-dots@OH were employed to monitor the intracellular temperature (32–42 °C), showing a clear trend for temperature variation in a single cell, indicating that C-dots could offer a powerful tool for a potential precise measurement of the intracellular temperature. They could also be used as thermal sensors for nano-electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

Luminescent solar concentrators (LSCs) are large-area sunlight collectors, consisting of a waveguide embedded with fluorophores. LSCs could reduce the use of expensive silicon solar cells, thus decreasing the cost of electricity. Although great efforts have been made for fabricating high efficiency large-area LSCs, there is still a lack of knowledge of the temperature effect on the performance of the LSCs based on colloidal quantum dots (QDs) because the LSCs need to be operated in outdoor conditions. In this work, we investigated the thermal effect on the performance of the LSCs based on colloidal QDs upon sunlight irradiation under different temperatures (10–40 °C). The optical properties (e.g. quantum yield and optical efficiency) of the LSCs are strongly dependent on the operating temperature. With increasing operating temperature, the quantum yield and optical efficiency decrease significantly. Among three types of configuration, the LSC based on thin-film QDs coated on glass has the lowest temperature increase under operation, due to the higher thermal conductivity of the glass compared to the polymer matrix. Considering the real operating conditions of the LSCs, the glass-based LSCs have promising potential for future high-efficiency LSC-PV systems. [ABSTRACT FROM AUTHOR]

Colloidal quantum dot (QD)-based photoelectrochemical (PEC) cells are cost-effective devices showing remarkable solar-to-fuel conversion efficiency. However, the extensive use of highly toxic elements (e.g. Pb and Cd) in QDs' synthesis and device fabrication is still a major challenge towards their practical development. Herein, we fabricate a solar-driven PEC cell based on environmentally friendly Mn-alloyed CuInS2 (MnCIS)/ZnS core/shell QDs, showing more favorable band alignment, efficient charge transfer, reduced charge recombination and lower charge transfer resistance with respect to the control device fabricated using unalloyed CuInS2 (CIS)/ZnS core/shell QDs. An unprecedented photocurrent density of ∼5.7 mA cm−2 with excellent stability was obtained for the as-fabricated MnCIS/ZnS core/shell QD-based PEC device when operated under standard one sun irradiation (AM 1.5G, 100 mW cm−2). These results indicate that the transition metal-alloyed environmentally friendly core/shell QDs are promising for next-generation 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]

Hydrogen generation from water under sunlight illumination is the key to construct a sustainable and clean energy system. To date, the long-term stability of photoanodes based on colloidal quantum dots (QDs) for hydrogen generation remains a major challenge due to the degradation of the QDs and hole accumulation. The degradation is usually caused by the self-oxidation of QDs induced by ultraviolet radiation. Here, we propose a concept of integrating a luminescent solar concentrator (LSC) with a photoelectrochemical (PEC) cell into a standalone device to improve the stability of the PEC device for hydrogen generation by avoiding direct irradiation of ultraviolet light on PEC devices. In this study, CdSe/(CdSxSe1−x)5/CdS core/multi-shell QDs were used as a photosensitizer. The PEC device based on alloyed multi-shell QDs shows a saturated photocurrent density of 11.5 mA cm−2 and maintains ∼16.2% of its initial value after 23 hours of sunlight illumination (100 mW cm−2). PEC devices based on QDs coupled with an LSC exhibit a saturated photocurrent density of 1.2 mA cm−2, and ∼84.6% of its initial value was retained after 23 hours of continuous illumination, indicating a 420% enhancement compared to a PEC device alone. This finding proves a unique concept to improve the photo-stability of PEC devices by coupling with an LSC. [ABSTRACT FROM AUTHOR]

Mesoporous TiO2 sensitized with colloidal quantum dots (QDs) is considered as a promising system for photoelectrochemical (PEC) hydrogen generation, in view of its low cost and high solar energy to fuel conversion efficiency. On the other hand, the limited long term stability and low current density of this system still hinder its commercialization. Here, we report a CdSe/CdSeS alloy/CdS core/shell/shell QD sensitized mesoporous TiO2 photoanode, which exhibits high performance and long-term stability for solar-driven hydrogen generation. A gradient CdSe/alloy-shell/CdS core/shell/shell structure is designed to accelerate exciton separation through the band engineering approach. Compared with the common CdSe/CdS core/shell structure, light absorption of QDs containing an intermediate alloyed layer is extended to longer wavelengths and more importantly, the photocurrent density is improved up to 17.5 mA cm−2 under one sun illumination (AM 1.5 G, 100 mW cm−2), a record value for PEC cells based on colloidal QDs for hydrogen generation. In addition, the as-fabricated PEC cell shows an unprecedented long-term stability, maintaining 50% of its initial value after continuous operation for over 39 hours, indicating that the gradient core/shell/shell QD based photoanode is a promising candidate for solar-driven hydrogen generation. [ABSTRACT FROM AUTHOR]

Luminescent solar concentrators (LSCs) serve as large-area sunlight collectors, which are suitable for applications in semi-transparent, low-cost photovoltaics. Colloidal quantum dots (QDs) are promising candidates as a new type of absorber/emitter in LSCs, due to their size-tunable wide absorption spectrum, narrow emission spectrum, high quantum yield and structure-engineered large Stokes shift. However, due to the lack of long-term photo-stability of QDs, it is still a challenge to prepare photostable LSCs. Compared with colloidal QDs, carbon dots (C-dots) exhibit excellent photo-stability, presenting a very promising solution to improve the long-term photo-stability of quantum dot based LSCs. Here, as a proof-of-concept, we report the enhancement of the photo-stability of LSCs based on CdSe/CdS QDs by using a tandem structure of LSCs. We fabricated a large-area (∼100 cm2) thin film and placed this thin film on the top of a luminescent solar concentrator (LSC) based on CdSe/CdS QDs. The as-fabricated semi-transparent tandem device exhibits an external optical efficiency of ∼1.4% under one sun illumination, which is a 16% enhancement in the efficiency over the LSC based on CdSe/CdS QDs. More importantly, the presence of C-dots largely enhances the photo-stability of the LSC device based on CdSe/CdS QDs under ultraviolet (UV) illumination. After 70 hour UV illumination, the QD-based LSC with a protective layer of C-dot thin film retains 75% of its initial integrated photoluminescence intensity, which is 1.8 times higher than that of the LSC without the C-dot layer. Our work indicates that C-dots can be used as an additional layer in LSCs to enhance both the efficiency and long-term photo-stability. [ABSTRACT FROM AUTHOR]

Luminescent solar concentrators (LSCs) can serve as large-area sunlight collectors, are suitable for applications in high-efficiency and cost-effective photovoltaics (PVs), and provide adaptability to the needs of architects for building-integrated PVs, which makes them an attractive option for transforming buildings into transparent or non-transparent electricity generators. Compared with traditional organic dyes, colloidal semiconducting quantum dots (QDs) are excellent candidates as emitters for LSCs because they exhibit wide size/shape/composition-tunable absorption spectra ranging from ultraviolet to near infrared, significantly overlapping with the solar spectrum. They also feature narrow emission spectra, high photoluminescence quantum yields, high absorption coefficients, solution processability and good photostability. Most importantly, QDs can be engineered to provide a minimal overlap between absorption and emission spectra, which is key to the realization of large-area LSCs with largely suppressed reabsorption energy losses. In this review article, we will first present and discuss the working principle of LSCs, the synthesis of colloidal QDs using wet-chemistry approaches, the optical properties of QDs, their band alignment and the intrinsic relationship between the band energy structure and optical properties of QDs. We focus on emerging architectures, such as core/shell QDs. We then highlight recent progress in QD-based LSCs and their anticipated applications. We conclude this review article with the major challenges and perspectives of LSCs in future commercial technologies. [ABSTRACT FROM AUTHOR]

The long term stability of photoelectrochemical (PEC) devices based on colloidal quantum dots (QDs) for hydrogen production is a major challenge. The degradation is often caused by the self-oxidation of QDs induced by the excess accumulation of holes in the valance band. Here, we use heterostructured “giant” core/alloyed-shell CdSe/PbxCd1−xS/CdS QDs to sensitize TiO2 mesoporous films for PEC hydrogen production. Transient fluorescence analysis results show that the use of a PbxCd1−xS gradient layer leads to a three-fold increase in the hole transfer rate compared to a pure CdS shell with similar shell thickness. The as-prepared PEC cell using alloyed shell “giant” QDs exhibits an enhanced photocurrent density of 10.2 mA cm−2 (97 mL per cm2 per day) under one sun illumination (100 mW cm−2). The PEC cell based on alloyed shell “giant” QDs shows an enhanced PEC device stability, with retention of ∼94.9% of its initial photocurrent after 2 h under one sun illumination. This finding provides unique insights to improve the stability and functional performance of PEC devices in which the photoanode is sensitized using colloidal “giant” QDs. [ABSTRACT FROM AUTHOR]

Synthesis of quantum dots (QDs) with widely size-tunable optical absorption and high photoluminescence quantum yield (PL QY) via a facile route is highly desired. By introducing tributylphosphine (TBP) into a relatively green synthesis method based on the use of S, PbCl2 and oleylamine (OLA), we conveniently synthesized ultrasmall PbS QDs with the first excitonic absorption peak wavelength as short as 705 nm, without using a glove box, which cannot be achieved by previously reported approaches, without involving smelly S precursors (such as bis(trimethylsilyl) sulfide). Such synthesized PbS QDs show narrow size distributions without any aggregation and demonstrate high PL QY in the range of 60–90%, depending on the QD size. Based on nuclear magnetic resonance spectroscopy and X-ray diffraction investigations, TBP was found to act as the passivation ligand on the surface of QDs while simultaneously assisting the transformation of PbCl2–OLA into more reactive Pb(OH)Cl that can directly participate the nucleation process, yielding ultrasmall PbS QDs. This new finding renders Pb(OH)Cl a very promising, new lead precursor for convenient synthesis of PbS and other lead-based QDs. We also demonstrate that the process can be readily scaled up. After synthesizing a thin CdS shell (∼0.1 nm), ultrasmall core/shell QDs with a large Stokes shift (0.36 eV) and good stability were employed for fabricating near infrared (NIR) luminescent solar concentrators, which led to a record-high optical efficiency of ∼1.2% at a geometric factor of ∼50 (10 cm in length). The TBP route developed herein is very promising for synthesizing high quality ultrasmall QDs that have high potential in NIR-related applications. [ABSTRACT FROM AUTHOR]

In this study, we report anomalous size-dependent photoluminescence (PL) intensity variation of PbS quantum dots (QDs) with the formation of a thin CdS shell via a microwave-assisted cation exchange approach. Thin shell formation has been established as an effective strategy for increasing the PL of QDs. Nonetheless, herein we observed an unusual PL decrease in ultrasmall QDs upon shell formation. We attempted to understand this abnormal phenomenon from the perspective of trap density variation and the probability of electrons and holes reaching surface defects. To this end, the quantum yield (QY) and PL lifetime (on the ns–μs time scales) of pristine PbS QDs and PbS/CdS core/shell QDs were measured and the radiative and non-radiative recombination rates were derived and compared. Moreover, transient absorption (TA) analysis (on the fs–ns time scale) was performed to better understand exciton dynamics at early times that lead to and affect longer time dynamics and optical properties such as PL. These experimental results, in conjunction with theoretical calculations of electron and hole wave functions, provide a complete picture of the photophysics governing the core/shell system. A model was proposed to explain the size-dependent optical and dynamic properties observed. [ABSTRACT FROM AUTHOR]

We report on the effects of incorporation of different concentrations of carboxyl group (COOH)-functionalized multi-wall carbon nanotubes (F-MWCNTs) into TiO2 active layers for dye-sensitized solar cells (DSSCs). Standard DSSCs with bare TiO2 exhibit a photo-conversion efficiency (PCE) of 6.05% and a short circuit current density (Jsc) of 13.3 mA cm−2. The presence of 2 wt% F-MWCNTs in the photoanodes increases the PCE up to 7.95% and Jsc up to 17.5 mA cm−2. The photoanodes were characterized using scanning electron microscopy (SEM), Fourier transform infrared (FTIR) spectroscopy and Raman spectroscopy. The electrochemical behaviour of the solar cells was investigated by electrochemical impedance spectroscopy (EIS). We attribute the improved performances to the combined effect of increased dye loading and reduced charge recombination (as clarified by dye loading and EIS measurements), due to the conformal coverage of F-MWCNTs, which allows fast and efficient charge collection in operating solar cells. These results can help in improving the PCE in DSSCs in an elegant and straightforward way, minimizing the need of additional steps (e.g. pre- and post-treatment with TiCl4) for photoanode preparation. [ABSTRACT FROM AUTHOR]

Electrophoretic deposition (EPD) is a simple technique for the uptake of nanoparticles into mesoporous films, for example to graft semiconducting nanocrystals (quantum dots, QDs) on mesoporous oxide thick films acting as photoanodes in third generation solar cells. Here we study the uptake of colloidal QDs into mesoporous TiO2 films using EPD. We examined PbS@CdS core@shell QDs, which are optically active in the near infrared (NIR) region of the solar spectrum and exhibit improved long-term stability toward oxidation compared to their pure PbS counterpart, as demonstrated by X-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) spectroscopy. We applied Rutherford backscattering spectrometry (RBS) to obtain the Pb depth profile into the TiO2 matrix. EPD duration in the range from 5 to 120 min and applied voltages from 50 to 200 V were considered. The applied electric field induces the fast anchoring of QDs to the oxide surface. Consequently, QD concentration in the solution contained in the mesoporous film drastically decreases, inducing a Fick-like diffusion of QDs. We modelled the entire process as a QD diffusion related to the formation of a QD concentration gradient, and a depth-independent QD anchoring, and were able to determine the electric field-induced diffusion coefficient D for QDs and the characteristic time for QD grafting, in very good agreement with the experiment. D increases from (1.5 ± 0.4) × 10-5 μm2 s-1 at 50 V to (1.1 ± 0.3) × 10-3 μm2 s-1 at 200 V. The dynamics of EPD may also be applied to other different colloidal QDs and quantum rod materials for the sensitization of mesoporous films. These results quantitatively describe the process of QD uptake during EPD, and can be used to tune the optical and optoelectronic properties of composite systems, which determine, for instance, the photoconversion efficiency in QD solar cells (QDSCs). [ABSTRACT FROM AUTHOR]

Cation exchange, recently explored for synthesizing core/shell quantum dots (QDs), causes continuous core size change during shell formation. By carefully varying parent PbS QD size and cation exchange conditions, we have synthesized PbS/CdS core/shell QDs with a similar PbS core size of ∼4.5 nm yet a different CdS shell thickness. This enables us to study the effect of shell thickness on the properties of PbS QDs after their transfer from chloroform into water viapoly(maleic anhydride-alt-1-octadecene-co-poly(ethylene glycol)). It was found that the quantum yield (QY) of PbS cores in water firstly increases with shell thickness up to ∼0.7 nm, reaching 33%, owing to better surface passivation and then decreases to 1.7% when the shell thickness reaches 2.3 nm. Such decline is due to the formation of new defects with shell deposition. In contrast, the variation amplitude of QY during water transfer monotonically decreases and QD photostability monotonically improves with shell thickness. It is clear that although newly introduced defects play a fundamental role in the absolute QY, they do not show any overwhelmingly negative effects on the variation of QY with environments and photostability. The colloidal stability of QDs in buffers containing different salt concentrations seems to be not affected by the shell thickness, possibly due to the same steric stabilization effect of the amphiphilic polymer in all samples. Further investigation on a series of core/shell samples confirms that ∼0.7 nm is an optimal shell thickness for various core sizes investigated herein, consistently yielding the maximum QY and reasonably good photostability. [ABSTRACT FROM AUTHOR]

Self-selected recovery of the photoluminescence (PL) of amphiphilic polymer encapsulated PbS quantum dots (QDs) was observed in water for the first time and possible mechanisms were proposed based on investigations by means of transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction and fluorescence spectroscopy. Water-soluble PbS QDs were synthesized by transferring monodispersed QDs capped with hydrophobic ligands of oleylamine from an organic solvent into water viaamphiphilic polymers poly(maleic anhydride-alt-1-octadecene-co-poly(ethylene glycol)). The water transfer process leads to a double size distribution (5.6 ± 0.9 nm and 2.7 ± 0.4 nm), attributed to ligand etching together with Ostwald ripening, as well as the fast decay of PL. The automatic recovery of the PL in PbS QDs stored in water in the dark for 3 months was only observed for the subset of smaller QDs and is largely due to the removal of surface defects with aging, as evidenced by the decreased percentage of unpassivated surface atoms from XPS studies. In contrast, the PL of the subset of larger QDs in the same sample does not self-recover in water and can only be slightly recovered by transferring them into environments with less external quenches. The results strongly suggest that it is the surface defect in the larger QDs themselves, introduced during Ostwald ripening, that is primarily responsible for their non-emitting status or rather low PL intensity under different conditions. The increase of unpassivated Pb atoms in larger PbS QDs after the 3 month aging has been confirmed by XPS, which explains their non-recovery behavior in water. The PL-recovered QD sample in water is very stable and shows comparable photostability to the initial QDs dispersed in an organic phase. [ABSTRACT FROM AUTHOR]

High-quality water-soluble near-infrared emitting quantum dots (QDs) are synthesized using a two-step approach for the first time. The CdS shell efficiently increases the structural stability of PbS QDs during water transfer and leads to good photostability and a significantly enhanced quantum yield as high as 30% in buffer. [ABSTRACT FROM AUTHOR]

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 cm 2 ) with a laminated structure based on the yellow emitting CDs achieves an optical conversion efficiency ( η opt ) of 4.56% and power conversion efficiency ( η PCE ) 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.

Compared to noble metals, transition metal oxides (TMOs) have positive development prospects in the field of electrocatalysis, and the synergy between the elements in multi-element TMO-based materials can improve their catalytic activity. However, it is still a challenge to synthesize multi-component TMO-based catalysts and deeply understand the effects of components on the catalytic performance of the catalysts. Here, we demonstrate multi-element ultra-small-sized nanofibers for efficient hydrogen production. The ternary NiFeCoO nanofiber-based electrode reached an overpotential of 82 mV at the current density of 10 mA cm -2 with a Tafel slope of 56 mV dec -1 in 1 M KOH, which are close to those of Pt plate (66 mV at 10 mA cm -2 ; the Tafel slope is 32 mV dec -1 ). In addition, the current density maintained 97% of its initial value after 10 h operation. We used the ternary NiFeCoO nanofiber-based electrode as an efficient counter electrode in photoelectrochemical hydrogen production to demonstrate the versatility of these nanofibers., Competing Interests: The authors declare no competing interests., (This journal is © The Royal Society of Chemistry.)

Luminescent solar concentrators (LSCs) are light-weight, semitransparent and large-area sunlight collectors for solar-to-electricity conversion. To date, carbon quantum dots (C-QDs) have attracted a lot of attention due to their size/shape/composition tunable optical properties, high quantum yield, excellent photostability, lower toxicity and simple synthetic methods using earth-abundant and low-cost precursors. However, due to the overlap between their absorption and emission spectra, it is still challenging to fabricate high-efficiency LSCs based on C-dots. In this work, we used C-QDs to fabricate semi-transparent large-area laminated LSCs (10 × 10 cm 2 ). C-QDs have the absorption spectrum ranging from 300 to 550 nm with a Stokes shift of 0.6 eV. By optimizing the concentration of C-QDs, the laminated LSC exhibits a highest η opt of 1.6%, which is 1.6 times higher than that of a single-layer LSC (100 mW cm -2 ). In addition, the laminated LSC exhibits a power conversion efficiency of 0.7% under natural sunlight illumination (62 mW cm -2 ) with excellent photostability. These findings suggest that laminated structured LSCs could be used for efficient solar energy harvesting compared to single layer or tandem structured LSCs based on colloidal C-QDs., Competing Interests: The authors declare no conflict of interest., (This journal is © The Royal Society of Chemistry.)

Solar-driven photoelectrochemical (PEC) hydrogen (H 2 ) 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 H 2 generation, by fabricating a hybrid photoanode obtained by incorporating small amounts of multiwall carbon nanotubes (MWCNTs) into a TiO 2 mesoporous film and sensitizing with colloidal heterostructured CdSe/(CdSe x S 1-x ) 5 /(CdS) 2 quantum dots (QDs). The latter were specially designed to accelerate the exciton separation through a band engineering approach. The PEC devices based on the TiO 2 /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 V RHE ) under one sun illumination (AM 1.5G, 100 mW cm -2 ), which is 40% higher than that of the reference device based on TiO 2 /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 TiO 2 -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.