Development of Nonstoichiometric CuInS₂ as a Light-Harvesting Photoanode and Catalytic Photocathode in a Sensitized Solar Cell

A simple one-pot approach was developed to obtain nonstoichiometric CuInS₂ nanocrystals. Using this approach, both In-rich and Cu-rich CuInS₂ nanocrystals could be reliably synthesized by tuning stoichiometric combinations of [Cu]/[In] precursor constituents. By designing Cu-rich CuInS₂ heteronanostructures to serve as counter electrodes, quantum-dot-sensitized solar cells (QDSSCs) equipped with In-rich CuInS₂ and CdS cosensitizers delivered a power conversion efficiency of 2.37%, which is significantly more efficient than conventional Pt counter electrodes. To the best of our knowledge, this study represents the first report utilizing nonstoichiometric CuInS₂ nanocrystals as a photon-harvesting sensitizer comprised of a photoanode and photocathode in QDSSCs; also unique to this report, these nonstoichiometric CuInS₂ nanocrystals were formed by simply changing the cationic molar ratios without complicated precursor preparation. Impedance spectroscopy and Tafel polarization indicated that these Cu-rich CuInS₂ heteronanostructures had electrocatalytic activities (used for reducing S^2–/S_<i>n</i>^2–) that were superior to a Pt catalyst. Moreover, we demonstrated that Cu-rich CuInS₂ heteronanostructures were also useful counter electrodes in dye-sensitized solar cells, and this finding revealed a promising conversion efficiency of 6.11%, which was ∼96% of the efficiency in a cell with a Pt-based counter electrode (6.32%)

Improved Performance of CuInS₂ Quantum Dot-Sensitized Solar Cells Based on a Multilayered Architecture

This Article describes a CuInS₂ quantum dot (QD)-sensitized solar cell (QDSSC) with a multilayered architecture and a cascaded energy-gap structure fabricated using a successive ionic-layer adsorption and reaction process. We initially used different metal chalcogenides as interfacial buffer layers to improve unmatched band alignments between the TiO₂ and CuInS₂ QD sensitizers. In this design, the photovoltaic performance, in terms of the short-circuit current density (<i>J</i>_SC), open-circuit voltage (<i>V</i>_OC), fill factor (FF), and power conversion efficiency (PCE), was significantly improved. Both <i>J</i>_SC and <i>V</i>_OC were improved in CuInS₂-based QDSSCs in the presence of interfacial buffer layers because of proper band alignment across the heterointerface and the negative band edge movement of TiO₂. The PCE of CuInS₂-based QDSSCs containing In₂Se₃ interfacial buffer layers was 1.35%, with <i>J</i>_SC = 5.83 mA/cm², <i>V</i>_OC = 595 mV, and FF = 39.0%. We also examined the use of alternative CdS and CdSe hybrid-sensitized layers, which were sequentially deposited onto the In₂Se₃/CuInS₂ configuration for creating favorable cascaded energy-gap structures. Both <i>J</i>_SC (11.3 mA cm^–2) and FF (47.3%) for the CuInS₂/CdSe hybrid-sensitized cells were higher than those for CuInS₂-based cells (<i>J</i>_SC = 5.83 mA cm^–2 and FF = 39.0%). In addition, the hybrid-sensitized cells had PCEs that were 1.3 times those of cells containing identically pretreated In₂Se₃ interfacial buffer layers. Additionally, we determined that ZnSe served as a good passivation layer on the surface of CuInS₂/CdSe hybrid-sensitized QDs, prevented current leakage from the QDs to electrolytes, and lowered interfacial charge recombination. Under simulated illumination (AM 1.5, 100 mW cm^–2), multilayered QDSSCs with distinct architectures delivered a maximum external quantum efficiency of 80% at 500 nm and a maximum PCE of 4.55%, approximately 9 times that of QDSSCs fabricated with pristine CuInS₂

Synthesis of Eco-Friendly CuInS₂ Quantum Dot-Sensitized Solar Cells by a Combined Ex Situ/in Situ Growth Approach

A cadmium-free CuInS₂ quantum dot (QD)-sensitized solar cell (QDSC) has been fabricated by taking advantage of the ex situ synthesis approach for fabricating highly crystalline QDs and the in situ successive ionic-layer adsorption and reaction (SILAR) approach for achieving high surface coverage of QDs. The ex situ synthesized CuInS₂ QDs can be rendered water soluble through a simple and rapid two-step method under the assistance of ultrasonication. This approach allows a stepwise ligand change from the insertion of a foreign ligand to ligand replacement, which preserves the long-term stability of colloidal solutions for more than 1 month. Furthermore, the resulting QDs can be utilized as sensitizers in QDSCs, and such a QDSC can deliver a power conversion efficiency (PCE) of 0.64%. Using the SILAR process, in situ CuInS₂ QDs could be preferentially grown epitaxially on the pre-existing seeds of ex situ synthesized CuInS₂ QDs. The results indicated that the CuInS₂ QDSC fabricated by the combined ex situ/in situ growth process exhibited a PCE of 1.84% (short-circuit current density = 7.72 mA cm^–2, open-circuit voltage = 570 mV, and fill factor = 41.8%), which is higher than the PCEs of CuInS₂ QDSCs fabricated by ex situ and in situ growth processes, respectively. The relative efficiencies of electrons injected by the combined ex situ/in situ growth approach were higher than those of ex situ synthesized CuInS₂ QDs deposited on TiO₂ films, as determined by emission-decay kinetic measurements. The incident photon-to-current conversion efficiency has been determined, and electrochemical impedance spectroscopy has been carried out to investigate the photovoltaic behavior and charge-transfer resistance of the QDSCs. The results suggest that the combined synergetic effects of in situ and ex situ CuInS₂ QD growth facilitate more electron injection from the QD sensitizers into TiO₂

Anchoring Ultrafine ZnFe₂O₄/C Nanoparticles on 3D ZnFe₂O₄ Nanoflakes for Boosting Cycle Stability and Energy Density of Flexible Asymmetric Supercapacitor

Heterostructure-based metal oxide thin films are recognized as the leading material for new generation, high-performance, stable, and flexible supercapacitors. However, morphologies, like nanoflakes, nanotubes, nanorods, and so forth, have been found to suffer from issues related to poor cycle stability and energy density. Thus, to circumvent these problems, herein, we have developed a low-cost, high surface area, and environmentally benign self-assembled ZnFe₂O₄ nanoflake@ZnFe₂O₄/C nanoparticle heterostructure electrode via anchoring ZnFe₂O₄ and carbon nanoparticles using an in situ biomediated green rotational chemical bath deposition approach for the first time. The synthesized ZnFe₂O₄ nanoflake@ZnFe₂O₄/C nanoparticle heterostructure thin films demonstrate an excellent specific capacitance of 1884 F g^–1 at a current density of 5 mA cm^–2. Additionally, all solid-state flexible asymmetric supercapacitor devices were designed on the basis of ZnFe₂O₄ nanoflake@ZnFe₂O₄/C nanoparticle heterostructures as the negative electrode and reduced graphene oxide and energy density of 81 Wh kg^–1 at a power density of 3.9 kW kg^–1. Similarly, the asymmetric device exhibits ultralong cycle stability of 35 000 cycles by losing only 2% capacitance. The excellent performance of the device is attributed to the self-assembled organization of the heterostructures. Moreover, the in situ biomediated green strategy is also applicable for the synthesis of other metal oxide and carbon-based heterostructure electrodes

Toward the Facile and Ecofriendly Fabrication of Quantum Dot-Sensitized Solar Cells via Thiol Coadsorbent Assistance

This paper reports a facile and environmentally friendly approach to the preparation of highly efficient quantum dot-sensitized solar cells (QDSSCs) based on a combination of aqueous CuInS₂ quantum dots (QDs) and thiol coadsorbents. The photovoltaic properties of the QDSSCs were found to be dependent on the type and concentration of the thiol coadsorbent. The incorporation of thiol coadsorbents results in improved <i>J</i>_SC and <i>V</i>_OC because (1) they provide disulfide reductants during the QD sensitization process and (2) the coadsorbent molecules are anchored on the TiO₂ surface, thus affecting the movement of the conduction band of TiO₂. To the best of the our knowledge, this is the first demonstrated use of various thiol coadsorbents as reducing agents in the fabrication of high-efficiency QDSSCs. CuInS₂ QDSSCs fabricated with the assistance of thioglycolic acid coadsorbents exhibited efficiencies as high as 5.90%, which is 20 times higher than that of the control device without thiol coadsorbents (0.29%). In addition, the photovoltaic properties of a device fabricated using the colloidal CuInS₂ QDs coated with different bifunctional linkers were investigated for comparison. The versatility of this facile fabrication process was demonstrated in the preparation of solar cells sensitized with aqueous AgInS₂ or CdSeTe QDs. The AgInS₂ QDSSC showed a conversion efficiency of 2.72%, which is the highest reported for Ag-based metal sulfides QDSSCs thus far

Magnetofluorescent Carbon Dots Derived from Crab Shell for Targeted Dual-Modality Bioimaging and Drug Delivery

We propose a one-pot microwave-assisted pyrolysis method for fabrication of magnetofluorescent carbon quantum dots (MFCQDs), using a combination of waste crab shell and three different transition-metal ions, Gd³⁺, Mn²⁺, and Eu³⁺, referred to as Gd@CQDs, Mn@CQDs, and Eu@CQDs, respectively. Chitin from waste crab shell acted not only as a carbon source but also as a chelating ligand to form complexes with transition-metal ions. Gd@CQDs exhibited a high <i>r</i>₁ relaxivity of 4.78 mM^–1·s^–1 and a low <i>r</i>₂/<i>r</i>₁ ratio of 1.33, suggesting that they show excellent potential as a <i>T</i>₁ contrast agent. Mn@CQDs and Eu@CQDs showed high <i>r</i>₂ relaxivity values of 140.7 and 28.32 mM^–1·s^–1, respectively, suggesting their potential for use as <i>T</i>₂ contrast agents. Further conjugation of Gd@CQDs with folic acid (FA) enabled specific targeting to folate receptor-positive HeLa cells, as confirmed via in vitro magnetic resonance and fluorescence imaging. Doxorubicin (DOX) was selected as a model drug for conjugation with FA-Gd@CQDs. The as-prepared nanocomposites showed significantly higher cytotoxicity toward HeLa cells than free DOX. No apparent cytotoxicity was observed in vivo (zebrafish embryos) or in vitro (cell viability), suggesting that MFCQDs show potential for development as diagnostic probes or theranostic agents

Synthesis of Cisplatin(IV) Prodrug-Tethered CuFeS₂ Nanoparticles in Tumor-Targeted Chemotherapy and Photothermal Therapy

In this study, for the first time, CuFeS₂ nanocrystals were successfully prepared through a facile noninjection-based synthetic strategy, by reacting Cu and Fe precursors with dodecanethiol in a 1-octadecene solvent. This one-pot noninjection strategy features easy handling, large-scale production, and high synthetic reproducibility. Following hyaluronic acid (HA) encapsulation, CuFeS₂ nanocrystals coated with HA (CuFeS₂@HA) not only readily dispersed in water and showed improved biocompatibility but also possessed a tumor-specific targeting ability of cancer cells bearing the cluster determinant 44 (CD44) receptors. The encapsulated CuFeS₂@HA showed broad optical absorbance from the visible to the near-infrared (NIR) region and high photothermal conversion efficiencies of about 74.2%. They can, therefore, be utilized for the photothermal ablation of cancer cells with NIR light irradiation. In addition, toxicity studies in vitro (B16F1 and HeLa) and in vivo (zebrafish embryos), as well as in vitro blood compatibility studies, indicated that CuFeS₂@HA show low cytotoxicity at the doses required for photothermal therapy. More importantly, CuFeS₂@HA can be used as delivery vehicles for chemotherapy cisplatin­(IV) prodrug forming CuFeS₂@HA-Pt­(IV). Their release profile revealed pH- and glutathione-mediated drug release from CuFeS₂@HA-Pt­(IV), which may minimize the side effects of the drug to normal tissues during therapy. Subsequent in vitro experiments confirmed that the use of CuFeS₂@HA-Pt­(IV) provides an enhanced and synergistic therapeutic effect compared to that from the use of either chemotherapy or photothermal therapy alone

Detection of Arsenic(III) through Pulsed Laser-Induced Desorption/Ionization of Gold Nanoparticles on Cellulose Membranes

We have developed an assay based on gold nanoparticle-modified mixed cellulose ester membrane (Au NPs-MCEM) coupled with laser-induced desorption/ionization mass spectrometry (LDI-MS)-for the detection of arsenic­(III) ions (arsenite, AsO₂^–) in aqueous solution. When the Au NPs reacted with lead ions (Pb²⁺) in alkaline solution (5 mM glycine–NaOH, pH 12), Au–Pb complexes, PbO, and Pb­(OH) were formed immediately on the Au NP surfaces. The Pb species reacted rapidly with subsequently added AsO₂^– to form PbOAs₂O₃, (PbO)₂As₂O₃, and/or (PbO)₃As₂O₃ shells (2–5 nm) on the Au NPs’ surfaces. As a result, significant observable aggregation of the Au NPs occurred in the solution. This Pb²⁺/Au NP probe allowed the detection of AsO₂^– at concentrations as low as 0.6 μM with high selectivity (at least 100-fold over other anions and metal ions). To further improve the sensitivity, we prepared Au NPs-MCEM for the LDI-MS-based detection of AsO₂^– ions. The intensity of the signal for the [Pb]⁺ ions in the mass spectra increased when the Au NPs-MCEM reacted with AsO₂^–; in contrast, the intensity of the signal for [Au]⁺ ions decreased. Accordingly, the [Pb]⁺/[Au]⁺ peak ratio increased upon increasing the AsO₂^– concentration over the range from 10 nM to 10 μM. The limit of detection at a signal-to-noise ratio of 3 was 2.5 nM, far below the action level of As (133 nM, ca. 10 ppb) permitted by the US EPA for drinking water. Relative to other nanoparticle-based arsenic sensors, this approach is rapid, specific, and sensitive; in addition, it can be applied to the detection of AsO₂^– in natural water samples (in this case, streamwater, lake water, tap water, groundwater, and mineral water)