Tunable structural and optical properties of CuInS2 colloidal quantum dots as photovoltaic absorbers

Facile phase selective synthesis of CuInS₂ (CIS) nanostructures has been an important pursuit because of the opportunity for tunable optical properties of the phases, and in this respect is investigated by hot-injection colloidal synthesis in this study. Relatively monodispersed colloidal quantum dots (3.8–5.6 nm) of predominantly chalcopyrite structure synthesized at 140, 180 and 210 °C over 60 minutes from copper(ii) hexafluoroacetylacetonate hydrate and indium(iii) diethyldithiocarbamate precursors exhibit temperature-dependent structural variability. The slightly off-stoichiometric quantum dots are copper-deficient in which copper vacancies , indium interstitials , indium–copper anti-sites and surface trapping states are likely implicated in broad photoluminescence emission with short radiative lifetimes, τ₁, τ₂, and τ₃ of 1.5–2.1, 7.8–13.9 and 55.2–70.8 ns and particle-size dependent tunable band gaps between 2.25 and 2.32 eV. Further structural and optical tunability (E_g between 2.03 and 2.28 eV) is achieved with possible time-dependent wurtzite to chalcopyrite phase transformation at 180 °C likely involving a dynamic interplay of kinetic and thermodynamic factors.

We report the facile hot-injection colloidal synthesis of near-stoichiometric CuInS₂ quantum dots at varying reaction times and temperatures which exhibit both optical and structural tunability with implications for enhanced photovoltaic utility.     

Facile synthesis of g-C3N4 quantum dots/graphene hydrogel nanocomposites for high-performance supercapacitor

This work demonstrates a facile one-pot method for preparing graphitic carbon nitride (g-C₃N₄) quantum dots/graphene hydrogel (CNQ/GH) nanocomposites using a hydrothermal process, in which graphene sheets of a graphene hydrogel (GH) are decorated with g-C₃N₄ quantum dots (CNQDs) and have a 3D hierarchical and interconnected structure through a typical self-assembly process. The obtained CNQ/GH nanocomposite demonstrates improved electrochemical performances of a supercapacitor with a specific capacitance of 243.2 F g⁻¹ at a current density of 0.2 A g⁻¹. In addition, the fabricated symmetric supercapacitor (SSC) using CNQ/GH electrodes exhibits a high energy density of 22.5 W h kg⁻¹ at a power density of 250 W kg⁻¹ and a superior cycling stability with a capacitance retention of 89.5% after 15 000 cycles. The observed improvements in the electrochemical performance of CNQ/GH electrodes are attributed to the large surface area with abundant mesopores and various C–N bonds in CNQDs, which promote efficient ion diffusion of electrolyte and electron transfer and provide more active sites for faradaic reactions. These obtained results demonstrate a facile and efficient route to develop potential electrode materials for high-performance energy storage device applications.

This work demonstrates a facile one-pot synthesis of graphitic carbon nitride (g-C₃N₄) quantum dots/graphene hydrogel (CNQ/GH) nanocomposites using a hydrothermal process, which shows excellent electrochemical performances for supercapacitors.     

Multimodal optical studies of single and clustered colloidal quantum dots for the long-term optical property evaluation of quantum dot-based molecular imaging phantoms

Understanding the optical properties of clustered quantum dots (QDs) is essential to the design of QD-based optical phantoms for molecular imaging. Single and clustered core/shell colloidal QDs of dimers, trimers, and tetramers are self-assembled, separated, and preferentially collected using electrospray differential mobility analysis (ES-DMA) with electrostatic deposition. Multimodal optical characterization and analysis of their dynamical photoluminescence (PL) properties enables the long-term evaluation of the physicochemical and optical properties of QDs in a single or a clustered state. A multimodal time-correlated spectroscopic confocal microscope capable of simultaneously measuring the time evolution of PL intensity fluctuation, PL lifetime, and emission spectra reveals the long-term dynamic optical properties of interacting QDs in individual dimeric clusters of QDs. This new method will benefit research into the quantitative interpretation of fluorescence intensity and lifetime results in QD-based molecular imaging techniques. The process of photooxidation leads to coupling of the QDs in a dimer, leading to unique optical properties when compared to an isolated QD. These results guide the design and evaluation of QD-based phantom materials for the validation of the PL measurements for quantitative molecular imaging of biological samples labeled with QD probes.     

Facile synthesis of a carbon dots and silver nanoparticles (CDs/AgNPs) composite for antibacterial application

Bacterial infections can seriously harm human health, and the overuse of traditional antibiotics and antibacterial agents will increase the resistance of bacteria. Therefore, it is necessary to prepare a new kind of antibacterial material. In this work, a carbon dots and silver nanoparticles (CDs/AgNPs) composite has been synthesized in a one-step facile method without the introduction of toxic chemicals, wherein CDs could serve as a reducing and stabilizing agent. The CDs/AgNPs composite was characterized by UV-vis spectrophotometry, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM), which demonstrate that the silver nanoparticles were successfully synthesized in the composite. The zeta potential of the CDs/AgNPs composite was −15.3 mV, indicating that the composite possesses high stability. Furthermore, the composite also exhibited biocidal effects for both Gram-negative E. coli bacteria and Gram-positive S. aureus bacteria. Thus, the composite is considered to be of great potential in bactericidal and biomedical applications.

One-step facile synthesis of a carbon dots and silver nanoparticles (CDs/AgNPs) composite without the introduction of toxic chemicals.     

Photostimulated control of luminescence quantum yield for colloidal Ag2S/2-MPA quantum dots

In this paper, we present the results on photoinduced formation of colloidal Ag₂S quantum dots with sizes of 1.5–3 nm passivated by 2-mercaptopropionic acid (Ag₂S/2-MPA) in the presence of ethylene glycol. The synthetized colloidal Ag₂S/2-MPA QDs have NIR recombination luminescence with its maximum near 800 nm. The control of absorption and luminescence properties of the QDs is achieved by photoactivation. It is shown that photoexposure of colloidal solution of Ag₂S/2-MPA QDs leads to an increase in the QD size and monodispersity along side with the growth of the luminescence quantum yield from 1% to 7.9%. Enhancement of the luminescence quantum yield is accompanied by an increase in the average luminescence lifetime up to 190 ns, which is due to the blocking of the nonradiative recombination channel with the radiative recombination rate being (3–5.5) × 10⁵ s⁻¹. It is shown that the purification of the Ag₂S/2-MPA solution by a dialysis membrane from regenerated cellulose leads to an increase in the sample monodispersity, as well as stops the photoinduced growth of QDs, and also reduces the degradation of their photoluminescence.

In this paper, we present the results on photoinduced formation of colloidal Ag₂S quantum dots with sizes of 1.5–3 nm passivated by 2-mercaptopropionic acid (Ag₂S/2-MPA) in the presence of ethylene glycol.     

Facile one-step fabrication of upconversion fluorescence carbon quantum dots anchored on graphene with enhanced nonlinear optical responses

A novel nanocomposite hybrid, carbon quantum dots (CQD)/graphene oxide (GO), which combines the favorable optical properties of both its components, is synthesized by a facile one-step electrochemical method. Transmission electron microscopy, Raman spectroscopy, UV-vis spectroscopy, and fluorescence studies show that the CQDs uniformly attach on the GO surface, which enables highly efficient energy transfer between CQDs and GO. The nonlinear optical and optical limiting (OL) performances are investigated by the open-aperture Z-scan technique in the nanosecond regime using a laser with a wavelength of 532 nm. The as-prepared CQD/GO composite offers a significantly improved OL performance compared with GO because of the charge/energy transfer process between the CQDs and GO. The main contributors to the enhanced OL effect in the CQD/GO hybrid are a combination of nonlinear scattering and increased nonlinear absorption resulting from efficient charge/energy transfer at the CQD/GO interface.

A novel nanocomposite hybrid, carbon quantum dots (CQD)/graphene oxide (GO), which combines the favorable optical properties of both its components, is synthesized by a facile one-step electrochemical method.     

Facile one-step synthesis of quaternary AgInZnS quantum dots and their applications for causing bioeffects and detecting Cu2+

Water-soluble AgInZnS quantum dots (AIZS QDs) were synthesized with glutathione (GSH) as a stabilizer by a facile one-step method based on a hydrothermal reaction between the nitrate salts of the corresponding metals and sodium sulfide as a sulfide precursor at 110 °C. The optimal reaction conditions (temperature, time, pH, and the molar ratios of the precursors) were studied. According to the data from TEM, XPS, and XRD, AIZS QDs were characterized with excellent optical properties. The results showed that the aqueous-dispersible AIZS QDs were quasi-spherical and their average diameter was 3.51 nm. Furthermore, the cytotoxicity of AIZS QDs was investigated by microcalorimetry and microscopy techniques (confocal microscopy and TEM). The data revealed that AIZS QDs exhibited low toxicity, biocompatibility, and good water stability, due to which they could be used as a fluorescent probe for bioimaging and labeling. In addition, AIZS QDs could be used as a sensor to detect Cu²⁺ because the fluorescence of AIZS QDs was quenched by Cu²⁺.

Water-soluble AgInZnS quantum dots were synthesized with glutathione as a stabilizer by a facile one-step method based on a hydrothermal reaction at 110 °C. It exhibited excellent optical properties, which can be used as sensor to detect Cu²⁺.     

Facile synthesis of carbon nitride quantum dots as a highly selective and sensitive fluorescent sensor for the tetracycline detection

Enhanced blue fluorescent carbon nitride quantum dots (g-C₃N₄QDs) were synthesized by a simple solvothermal “tailoring” process from bulk g-C₃N₄ and analyzed by various characterization methods. The as-obtained g-C₃N₄QDs were successfully applied in the determination of tetracycline (TC) with a good linear relationship in the range of 0.23–202.70 μM. The proposed fluorescent sensor shows excellent stability, good repeatability, high selectivity and outstanding sensitivity to TC with a low detection limit of 0.19 μM. The fluorescence quenching mechanism of g-C₃N₄QDs with TC was mainly governed by static quenching and the inner filter effect. The method was successfully applied to monitor TC in tap water and milk powder samples.

The g-C₃N₄QDs were synthesized by a simple solvothermal “tailoring” process from bulk g-C₃N₄ which have a “strong quenching” behaviour in the presence of TC. The proposed fluorescent sensor has been successfully applied to detect TC in actual samples.     

Biomimetic colloidal photonic crystals by coassembly of polystyrene nanoparticles and graphene quantum dots

Biomimetic nanostructured materials with iridescent structural colors have attracted great attention due to their potential in photonic devices, materials science, and biomedical engineering. The technological applications of artificial photonic crystals (PCs), however, are often hindered by their low color visibility. Herein, we report colloidal PCs with enhanced color visibility through the coassembly of thioglycerol-modified graphene quantum dots (GQDs) into the close-packed array of polystyrene (PS) nanospheres. The enhanced polystyrene PCs were fabricated by both centrifugal sedimentation and drop-casting methods. The color visibility of the resulting PCs was found to be strongly dependent on the hydrothermal time (i.e., carbonization) and the doping concentrations of GQDs. The PCs with brilliant reflection colors with red, green and blue (RGB) regions have been achieved by controlling the size of the constituent PS nanoparticles. As a proof of concept for photonic ink applications, we demonstrated a number of photonic images with RGB colors on multiple substrates including paper, silicon wafer and glass. This work is expected to provide new insight into the development of emerging advanced photonic crystals with high color visibility for applications such as colloidal paints, textile fabrics, and wearable displays.

We reported colloidal PCs with enhanced color visibility through the coassembly of modified graphene quantum dots into the close-packed array of polystyrene nanoparticles.     

Influence of precursor ratio and dopant concentration on the structure and optical properties of Cu-doped ZnCdSe-alloyed quantum dots

Tunable copper doped Zn_1−xCd_xS alloy quantum dots (QDs) were successfully synthesized by the wet chemical method. A one-step method is developed to synthesize doped ternary QDs which is more preferable than a two-step method. The influence of experimental parameters like the Zn/Cd ratio and Cu dopant concentration has been investigated using various spectroscopic techniques like UV-visible, photoluminescence, X-ray diffraction and Raman spectroscopy. The absorption and emission properties can be tuned by changing the concentration of components of the ternary QDs. The high concentration of dopant completely quenched the emission of the ternary QDs. EDX gives confirmation of the elemental composition of the synthesized samples. The obtained results suggest the successful doping of the ternary QDs. Interestingly, the study results revealed that the crystal structure (ZB and/or WZ) and the dual emission of the Cu-doped Zn_1−xCd_xSe alloy QDs could be controlled by varying the dopant concentration and chemical composition of the host. Doping also leads to enhancement in emission properties and provides more stability to ternary QDs. The enhancement in the photoluminescence (PL) decay lifetime of Cu-doped ternary QDs can be advantageous for optoelectronic and biosensor applications.

Tunable copper doped Zn_1−xCd_xS alloy quantum dots (QDs) were successfully synthesized by the wet chemical method.     

Semiconductor quantum dots: synthesis and water-solubilization for biomedical applications

Background: Quantum dots (QDs) are generally nanosized inorganic particles. They have distinctive size-dependent optical properties due to their very small size (mostly < 10 nm). QDs are regarded as promising new fluorescent materials for biological labeling and imaging because of their superior properties compared with traditional organic molecular dyes. These properties include high quantum efficiency, long-term photostability and very narrow emission but broad absorption spectra. Objective/methods: Recent developments in synthesizing high quality semiconductor QDs (mainly metal-chalcogenide compounds) and forming biocompatible structures for biomedical applications are discussed in this paper. Results/conclusions: This information may facilitate the research to create new materials/technologies for future clinical applications.     

Hydrothermal synthesis of water-soluble Mn- and Cu-doped CdSe quantum dots with multi-shell structures and their photoluminescence properties

Optical properties of semiconductor quantum dots (QDs) can be tuned by doping with transition metal ions. In this study, water-soluble CdSe/ZnS:Mn/ZnS QDs with the core/shell/shell structure were synthesized through a hydrothermal method, in which the surface of the CdSe core was coated with a ZnS:Mn shell and ZnS capping shell. Herein, the CdSe core QDs were prepared first and then doped with Mn²⁺; therefore, the QD size and doping level could be controlled independently and interference from the self-purifying effect could be avoided. When CdSe cores with diameters less than 1.9 nm were used, Mn-related photoluminescence (PL) was observed as the main PL band, whereas the band-edge PL was mainly observed when larger CdSe cores were used. Furthermore, using ZnS:Cu as the doping shell layer, CdSe/ZnS:Cu/ZnS and ZnSe/ZnS:Cu/ZnS nanoparticles were successfully synthesized, and Cu-related PL was clearly observed. These results indicate that the core/shell/shell QD structure with doping in the shell layer is a versatile method for synthesizing doped QDs.

The core/shell/shell QD structure with doping in the shell layer is a versatile method for synthesizing doped QDs.     

Frontier challenges in doping quantum dots: synthesis and characterization

Impurity doping in semiconductor quantum dots (QDs) has numerous prospects in implementing and altering their properties and technologies. Herein, we review the state-of-the-art doping techniques arising from colloidal synthesis methods. We first discuss the advantages and challenges involved in doping; we then discuss various doping techniques, including clustering of dopants as well as expulsion out of the lattice due to self-purification. Some of these techniques have been shown to open up a new generation of robust doped semiconductor quantum dots with cluster-free doping which will be suitable for various spin-based solid-state device technologies and overcome the longstanding challenges of controlled impurity doping. Further, we discuss inhibitors such as defects, clustering and interfaces, followed by current open questions. These include pathways to obtain uniform doping in the required radial position with unprecedented control over the dopant concentration and the size of the QDs.

We discuss state-of-the-art doping strategies for colloidal quantum dots, their principle, advantages and challenges in implementing the strategies.     

Semiconductor quantum dots: synthesis and water-solubilization for biomedical applications

BACKGROUND: Quantum dots (QDs) are generally nanosized inorganic particles. They have distinctive size-dependent optical properties due to their very small size (mostly < 10 nm). QDs are regarded as promising new fluorescent materials for biological labeling and imaging because of their superior properties compared with traditional organic molecular dyes. These properties include high quantum efficiency, long-term photostability and very narrow emission but broad absorption spectra. OBJECTIVE/METHODS: Recent developments in synthesizing high quality semiconductor QDs (mainly metal-chalcogenide compounds) and forming biocompatible structures for biomedical applications are discussed in this paper. Results/conclusions: This information may facilitate the research to create new materials/technologies for future clinical applications.     

Facile synthesis of ZnO nanobullets by solution plasma without chemical additives

ZnO nano-bullets were synthesized using solution plasma from only Zn electrode in water without any chemical agents. In this sustainable synthesis system, the rapid quenching reaction at the interface between the plasma/liquid phases facilitates the fast formation of nano-sized materials. The coil-to-pin type electrode geometry, which overcomes the discharge interruption owing to the electrode gap broadening of the typical pin-to-pin type enables the synthesis of numerous nanomaterials through a stable discharge for 1 h. The as-prepared samples exhibited a high crystalline ZnO structure without post calcination, and the length and width were 71.8 and 29.1 nm, respectively. The main exposed facet of ZnO nano-bullets was the (100) crystal facet, but interestingly, the (101) facet was confirmed at the inclined surfaces in the edges. The (101) crystal facet has an asymmetric Zn and O atom arrangement, and it could result in a focused electron density area with relatively high reactivity. Therefore, ZnO nano-bullets are promising materials for applications in advanced technologies.

ZnO nano-bullets were synthesized using only Zn electrode and water by solution plasma and new electrode geometry improved discharge time up to 1 h.     

Multi-shelled ZnO decorated with nitrogen and phosphorus co-doped carbon quantum dots: synthesis and enhanced photodegradation activity of methylene blue in aqueous solutions

The presence of organic dyes in wastewater has posed a huge threat to aquatic life and human health. In this study, nitrogen and phosphorus co-doped carbon quantum dot (CQD)-decorated multi-shelled ZnO microsphere photocatalysts (NPCQD/ZnO) were obtained via a simple absorption process; ZnO was prepared by calcining carbon microspheres as the sacrificial template. The as-prepared NPCQD/ZnO showed an obvious multi-shelled structure with the nitrogen and phosphorus co-doped CQDs homogeneously attached onto the inner and outer shells of ZnO. According to the UV-Vis DRS results, all the co-doped, single-doped and undoped carbon quantum dots could enhance the efficiency of absorption of visible light and reduce the optical band gap. Furthermore, the PL characterization results showed that the NPCQD/ZnO composites had lowest fluorescence intensity because the decoration of ZnO with NPCQDs could effectively reduce the recombination rate of photogenerated electron–hole pairs in the ZnO semiconductor photocatalyst. Importantly, 2 g-NPCQD/ZnO composites exhibited higher photodegradation performance towards methylene blue (MB) than pure ZnO and even the newly reported series of ZnO catalysts under the same conditions. Moreover, the degradation obeyed the pseudo-first-order and Langmuir–Hinshelwood kinetics models with a reaction constant of 0.0725 min⁻¹, which was 1.05 times that of pure ZnO (0.0353 min⁻¹). The NPCQD/ZnO composites not only showed good photocatalytic performance, but also had excellent stability since the photocatalytic activity did not significantly decrease after five cycling tests. In addition, compared with single-doped and undoped carbon quantum dots, N and P co-doped carbon quantum dots have more significant efficiency for the modification of semiconductor photocatalysts. The present study shows that the CQD-decorated multi-shelled ZnO can be regarded as an excellent photocatalyst candidate in the field of water treatment. Moreover, this new concept is helpful in the controllable construction of other multi-shelled metal oxides decorated with co-doped carbon quantum dots with enhanced photocatalytic properties.

The presence of organic dyes in wastewater has posed a huge threat to aquatic life and human health.     

A glassware-free combinatorial synthesis of green quantum dots using bubble wrap

Here, we describe the use of commercially-available bubble wrap as the basis for the simple, cheap combinatorial exploration of the synthesis of brightly emitting core/shell quantum dots.

In this communication, we highlight the use of bubble wrap in the simple parallel synthesis of CuInS₂-based quantum dots with different optical properties, based on varying precursors concentrations.

New chemical compounds are discovered at an astonishing rate; in 2015, the Chemical Abstract Service recorded its 100 millionth compound. Despite this, the actual hardware associated with synthetic chemistry has evolved at a much slower rate. Most chemical glassware is based on borosilicate glass (developed in 1915), with quartz, actinic and PTFE-coated glass used for analysis and storage.¹ There have been few developments in glassware until recently, when the Cronin group reported 3D-printed acetoxy silicone reaction-ware that could be used for cluster synthesis, with the geometry of the reaction vessel dictating the actual reaction product.² Further advances include the use of 3D-printed polypropylene modules that were combined and used to synthesise a simple drug.³One of the key developments in how reactions are carried out was the emergence of combinatorial chemistry and parallel synthesis, providing the ability to implement numerous chemical reactions simultaneously. When applied to material science, the seminal studies in combinatorial materials chemistry explored the preparation of superconducting materials using vapour deposition masks, allowing a library density of 10 000 samples per square inch.⁴ The emergence of materials chemistry has expanded beyond simple deposition techniques, and there exists a requirement for parallel solution chemistries to be developed. The use of combinatorial chemistry in material and nanoparticle science is well-documented, with luminescent materials (in which we are primarily interested) representing only a small fraction of what can be explored.⁵ Undertaking such work can be, however, expensive, requiring a large initial outlay for robotics kit for example, which makes such chemistry prohibitive for some developing laboratories. Also, not all combinatorial experiments need a sample density in the thousands; for some, tens of outputs are sufficient to arrive at a suitable positive outcome. Similarly, solution-based material science cannot rely on vapour deposition techniques.An important nanomaterial developed during the last three decades are the quantum dots. A combinatorial approach to the synthesis of CdSe quantum dots has been reported although this method utilised several microreactors and is beyond most synthetic laboratories.⁶ Likewise, a high-throughput robotic system has been used to explore the reproducible synthesis of various luminescent nanoparticles, giving unrivalled control and optimisation of the reactions; again, the instruments used in such a study are equally unique.⁷Inspired by a report from the Whiteside group which outlined the use of bubble wrap to store liquids, culture bacteria and take optical and electrochemical measurements, we describe the use of commercial bubble wrap as a simple reaction vessel and demonstrate multiple simultaneous reactions as a proof-of-principle for its use in basic combinatorial-style chemistry.⁸ Bubble wrap, as highlighted by Bwambok et al., has a variety of positive attributes that makes it ideal for simple aqueous-based chemistry. Of particular note are the low cost, ease of manipulation and disposal and the density of bubbles (library density) of up to 5000 m⁻², making this system particularly attractive to resource-poor laboratories. We also note that bubble wrap has been used to culture cells, carry out optical trapping experiments, and the colorimetric sensing of glucose.^9–11In this report, we have initially chosen the synthesis of core/shell CuInS₂/ZnS quantum dots as an example of where multiple simultaneous, parallel reactions can optimise luminescent nanoparticle synthesis; specifically, the optimum core compositions and shell precursor concentrations to achieve the brightest materials. CuInS₂ quantum dots were chosen as they are quickly emerging as key materials for numerous applications as they do not contain heavy metals, yet exhibit excellent optical properties, notably towards the red end of the visible spectrum. Such ‘green’ quantum dots may represent the next generation of materials for use in solar and biological applications, although such ternary solid-state materials have a wide variety of phases and stoichiometries.¹² Also, for the most basic applications, the majority of quantum dots require an inorganic shell layer to resist oxidation and confine charge carriers. Shell deposition is not simple, and often requires numerous attempts to reach the optimum thickness (bright point).¹³ If a shell is too thin, then the charge carriers are not sufficiently confined; if the shell is too thick, then the layer can exhibit defects that reduce the emission intensity. There, therefore, exists a complicated array of variables (for example, core material composition and shell thickness) that requires optimisation for core/shell quantum dot preparation.Whilst numerous reports exist on the various synthesis methods of the Cu/In/S quantum dot system¹² and the numerous resulting phases and materials, a key variable has not been explored in as much depth – that is, the optimum reaction conditions for luminescent core/shell materials. Determining the preferred core/shell structure in typical glassware reactions can be laborious and expensive, requiring numerous extensive, individual experiments and it would be beneficial if this could be decided simply and rapidly. In this work we report the exploration of several core precursor ratios and a shelling reaction in a few simple steps; the resulting brightest materials can be easily determined optically and then analysed and confirmed spectroscopically. We also describe how, after using bubble wrap to determine the key reaction parameters, we then carry out the entire reaction, from core synthesis to shell deposition in one single bubble, negating the need for standard glassware.Initially, whilst referring to previous reports,¹⁴ we prepared core CuInS₂ particles with a variety of Cu : In molar ratios (1 : 5, 1 : 10, 1 : 20, 1 : 40, 1 : 80), by pre-mixing the precursors for each ratio in separate vials, followed by injection of the reagents into a series of adjacent individual bubbles (ca. 5 mL volume, Fig. 1A for scale) on a single sheet, followed by heating the sheet in a water bath at 85 °C for 60 minutes. From these experiments, we determined by spectrometer that a range of materials with differing band edges and emissive properties resulted, confirming the variety of optical properties in this system. Materials prepared with a precursor ratio of 1 : 5 displayed absorption spectra with no excitonic features, whilst all other materials exhibited an excitonic feature at ca. 450 nm, although to varying degrees (ESI Fig. S1^†). The emission spectra showed a similar variation, with the 1 : 5 (Cu : In) ratio materials showing no evidence of emission, whilst the 1 : 10 ratio material clearly displayed two features, at ca. 550 nm and 650 nm (ESI Fig. S2^†). These emission features are strongly related to vacancies and defects, with the emission towards the red end of the visible spectrum reportedly associated with copper vacancies and donor–acceptor recombination.¹⁵ Features at ca. 535 nm have previously been observed by Macdonald et al. in CuInS₂ prepared in water, and again attributed to defects.¹⁶ By increasing the Cu : In ratio, the feature at ca. 650 nm decreased whilst the feature at ca. 550 nm became predominant. Despite emission being detected by spectrometer, none of the particles visibly fluoresced in the bubble wrap under 365 nm excitation, consistent with materials which exhibit a low emission quantum yield. The absorption spectra obtained using the reported method was similar to those obtained using an earlier synthetic method which was utilised for this work; the features exhibited no excitonic features, with onsets of absorption at ca. 600 nm. Emission spectra for similar reactions in glassware exhibited maxima between ca. 550 nm and 650 nm for core/shell materials, whilst emission spectra obtained for materials prepared in bubble wrap exhibited emission between ca. 550 nm and ca. 700 nm, extended into the red by a further 50 nm.¹⁴Open in a separate windowFig. 1(A) Photograph of quantum dot-filled bubbles with a £1 coin for size comparison; (B) photograph of bubbles filled with CuInS2/ZnS quantum dots (prepared with differing Cu : In ratios as shown) after synthesis; (C) photograph of materials in previous figure under 365 nm excitation; (D) normalised absorption spectra of CuInS₂/ZnS quantum dots prepared at different Cu : In ratios; (E) normalised emission spectra of CuInS₂/ZnS quantum dots prepared at different Cu : In ratios. This figure shows the range of optical properties available by varying precursor conditions in a simple bubble wrap synthesis.We then extended the synthesis to the preparation of a series of CuInS₂/ZnS quantum dots in bubble wrap by repeating the core synthesis using differing Cu : In ratios as described above. Following this, a set amount of ZnS shell precursors were added to each bubble and once resealed, the sheet was then reheated to 85 °C for a further 60 minutes. The resulting array of CuInS₂/ZnS quantum dots exhibited different colours (Fig. 1B), which were luminescent when illuminated at 365 nm (Fig. 1C), clearly emitting at different wavelengths. It was observed that the deposition of a ZnS shell on CuInS₂ core nanoparticles prepared by different precursor ratios produced quantum dots of varying band edge positions as suggested by the difference in colour of the samples, and different emissive colours and brightness. The materials were analysed spectroscopically (Fig. 1D and E) and the range of absorption and emission wavelengths were confirmed. The absorption band edges of the core/shell materials lost the sharp profile and all excitonic features yet remained in the same approximate spectral region (normalised spectra, Fig. 1C). The most surprising results were observed in the emission spectra, where the core/shell sample with Cu : In ratio of 1 : 5 exhibited an emission profile at ca. 700 nm (Fig. 1E), whereas the core material alone did not display any emission. Notably, all emission appeared composed of a single feature with similar full width at half the maximum (FWHM) of ca. 110 nm, with no evidence of the previous secondary feature at ca. 550 nm, in agreement with previous studies which attributed the high energy emission suppression to zinc passivation of donor defect sites.^15,16 Increasing the Cu : In ratio resulted in a gradual blue shift in the emission maxima, to a minimum wavelength of ca. 550 nm for core/shell samples prepared with a 1 : 80 core Cu : In precursor ratio. By eye, it was determined that CuInS₂/ZnS nanoparticles with core Cu : In precursor ratios of 1 : 10 and 1 : 20 resulted in the brightest materials, although both emitted at different wavelengths. It should be stated that despite significant work into the origin of emission from Cu/In/S nanomaterials, no clear opinion has been reached; although it is clear that radiative emission does not originate from a simple quantum confined band edge; rather from an as-yet unconfirmed defect state(s).Once we had ascertained that addition of a ZnS shell to core materials prepared with a 1 : 5 precursor ratio resulted in a luminescent structure (whereas the core material alone was non-luminescent, as determined visually and spectroscopically), we explored the potential for further enhancing the emission by varying the amount of ZnS precursors, potentially providing a thicker shell and hence exploring the opportunity to uncover a bright point. As can be seen from Fig. 2A–D, varying the amounts of shell precursor with extended heating had minimal effect on spectral position or emission brightness. Spectroscopic examination of the materials prepared in the bubbles displayed similar absorption edges (all exhibiting the same band edge position with a slight suggestion of an excitonic feature at ca. 500 nm as shown in Fig. 2C) whilst emission spectra appeared to reduce slightly in intensity with the addition of shell precursors (Fig. 2D) whilst maintaining the same spectral position, showing no evidence of exciton leakage into the shell (Fig. 2D, inset).Open in a separate windowFig. 2(A) Photograph of CuInS₂ quantum dots after various volumes (numbers shown in mL) of ZnS precursor had been added; (B) photograph of materials in previous figure under 365 nm excitation; (C) normalised absorption spectra of CuInS₂/ZnS quantum dots prepared using different amounts of ZnS precursor; (D) emission spectra of CuInS₂/ZnS quantum dots prepared using different amounts of ZnS precursor. Inset, normalised spectra showing spectral position. The data shows that addition of varying amounts of shell precursor had little effect on the optical properties of CuInS₂/ZnS.To ensure that the potential of bubble wrap synthesis extends beyond simple combinatorial-style synthetic experiments (where starting materials were injected from a precursor reservoir into numerous bubbles) and to mimic typical glassware-based reaction where precursors are sequentially injected, we prepared CuInS₂ quantum dot with a Cu : In ratio of 1 : 10, followed by addition of 0.2 mL ZnS precursor solution, entirely in a single bubble in a stand-alone experiment by sequentially injecting precursors and heating. In this case, a phosphate buffer solution of the metal precursors and capping agent (l-cysteine) were injected into a bubble then sealed. Following this, the sulfur precursor was injected and the bubble resealed, left for one hour, then heated in a water bath at 85 °C for one hour. Shell addition as described above, completed the reaction, the product of which was found to brightly luminesce when excited at 365 nm (Fig. 3A and B). Spectroscopic analysis of two identical reactions in adjacent bubbles confirmed almost identical optical band edges, whilst the emission profiles shifted position slightly (Fig. 3C and D). This confirms that pre-mixing precursors before addition to a bubble is unnecessary and that bubble wrap can be used as a stand-alone reaction vessel. Whilst the method can be easily adapted to aqueous-based quantum dot synthesis, there are obvious limitations. This method cannot, as yet, be used to make quantum dots at high temperatures in organic solvents, which is considered to be the most popular method. There are also no facilities to involve mixing, although this has not been an issue to date and we assume convective heating dominates solvent volumes as small as 5 mL, although this is clearly one factor that could impact which nanomaterial product is obtained; such issues have been previously highlighted as important in the high temperature synthesis of cobalt nanoparticles.¹⁷ Also, this method relies on the initial analysis of colours and intensities by eye, with no correction for mistaking optical density/colour or consideration for the eye''s response to green over red, for example. One should note however, that the ultimate use for these particles may be displays, which also rely on the eye''s inherent optical response. Likewise, optically-based assays have been developed that allow antigen detection at the femto-gram per milliliter level by the naked eye.¹⁸Open in a separate windowFig. 3(A) Photograph of two sets of CuInS₂/ZnS quantum dots prepared by a stand-alone method in two adjacent bubbles using the optimised conditions (Cu : In ratio of 1 : 10, followed by addition of 0.2 mL ZnS precursor solution); (B) photograph of CuInS₂/ZnS quantum dots used in previous figure excited at 365 nm; (C) normalised absorption spectra of the two samples shown in previous figures; (D) normalised emission spectra of the two samples shown in previous figures. This data shows that CuInS₂/ZnS quantum dots can be simply prepared using sequential precursor addition in a bubble, and that the method is reproduceable.In conclusion, we have demonstrated that bubble wrap can be used in a combinatorial style set of experiments to determine the optimum reaction conditions for brightly luminescent core/shell quantum dots. We also demonstrated that bubble wrap can be used as a stand-alone reaction vessel, withstanding numerous reagent injections and heating, which should be applicable to other simple chemical reactions.     

Facile one-pot synthesis of heterostructure SnO2/ZnO photocatalyst for enhanced photocatalytic degradation of organic dye

In this work, heterostructure SnO₂/ZnO nanocomposite photocatalyst was prepared by a straightforward one step polyol method. The resulting photocatalysts were characterized by X-ray diffraction (XRD), nitrogen adsorption–desorption analyses, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and UV-vis diffuse reflectance spectroscopy (UV-vis DRS). The results showed that the synthesized SnO₂/ZnO nanocomposites possessed mesoporous wurtzite ZnO and cassiterite SnO₂ nanocrystallites. The photocatalytic activity of the prepared SnO₂/ZnO photocatalyst was investigated by the degradation of methylene blue dye under UV light irradiation. The heterostructure SnO₂/ZnO photocatalyst showed much higher photocatalytic activities for the degradation of methylene blue dye than individual SnO₂, ZnO nanomaterials and reference commercial TiO₂ P25. This higher photocatalytic degradation activity was due to enhanced charge separation and subsequently the suppression of charge recombination in the SnO₂/ZnO photocatalyst resulting from band offsets between SnO₂ and ZnO. Finally, these heterostructure SnO₂/ZnO nanocatalysts were stable and could be recycled several times without any appreciable change in degradation rate constant which opens new avenues toward potential industrial applications.

In this work, heterostructure SnO₂/ZnO nanocomposite photocatalyst was prepared by a straightforward one step polyol method.