Fortification of CdSe Quantum Dots with Graphene Oxide. Excited State Interactions and Light Energy Conversion

Graphene based 2-D carbon nanostructures provide new opportunities to fortify semiconductor based light harvesting assemblies. Electron and energy transfer rates from photoexcited CdSe colloidal quantum dots (QDs) to graphene oxide (GO) and reduced graphene oxide (RGO) were isolated by analysis of excited state deactivation lifetimes as a function of degree of oxidation and charging in (R)­GO. Apparent rate constants for energy and electron transfer determined for CdSe–GO composites were 5.5 × 10⁸ and 6.7 × 10⁸ s^–1, respectively. Additionally, incorporation of GO in colloidal CdSe QD films deposited on conducting glass electrodes was found to enhance the charge separation and electron conduction through the QD film, thus allowing three-dimensional sensitization. Photoanodes assembled from CdSe–graphene composites in quantum dot sensitized solar cells display improved photocurrent response (∼150%) over those prepared without GO

<i>Sun-Believable</i> Solar Paint. A Transformative One-Step Approach for Designing Nanocrystalline Solar Cells

A transformative approach is required to meet the demand of economically viable solar cell technology. By making use of recent advances in semiconductor nanocrystal research, we have now developed a one-coat solar paint for designing quantum dot solar cells. A binder-free paste consisting of CdS, CdSe, and TiO₂ semiconductor nanoparticles was prepared and applied to conducting glass surface and annealed at 473 K. The photoconversion behavior of these semiconductor film electrodes was evaluated in a photoelectrochemical cell consisting of graphene–Cu₂S counter electrode and sulfide/polysulfide redox couple. Open-circuit voltage as high as 600 mV and short circuit current of 3.1 mA/cm² were obtained with CdS/TiO₂–CdSe/TiO₂ electrodes. A power conversion efficiency exceeding 1% has been obtained for solar cells constructed using the simple conventional paint brush approach under ambient conditions. Whereas further improvements are necessary to develop strategies for large area, all solid state devices, this initial effort to prepare solar paint offers the advantages of simple design and economically viable next generation solar cells

Electron Hopping Through Single-to-Few-Layer Graphene Oxide Films. Side-Selective Photocatalytic Deposition of Metal Nanoparticles

Single- to few-layer graphene oxide (GO) sheets have been successfully anchored onto TiO₂ films using electrophoretic deposition. Upon UV illumination of TiO₂–GO films, photogenerated electrons from TiO₂ are captured by GO. These electrons are initially used in GO’s reduction, while additional electron transfer results in storage across its sp² network. In the presence of silver ions, deposition of silver nanoparticles (NPs) is accomplished on the GO surface opposite the TiO₂, thus confirming the ability of GO to transport electrons through its plane. Illumination-controlled reduction of silver ions allows for simple selection of particle size and loading, making these semiconductor–graphene–metal (SGM) films ideal for custom catalysis and sensor applications. Initial testing of SGM films as surface-enhanced resonance Raman (SERRS) sensors produced significant target molecule signal enhancements, enabling detection of nanomolar concentrations

Electron Hopping Through Single-to-Few-Layer Graphene Oxide Films. Side-Selective Photocatalytic Deposition of Metal Nanoparticles

Single- to few-layer graphene oxide (GO) sheets have been successfully anchored onto TiO₂ films using electrophoretic deposition. Upon UV illumination of TiO₂–GO films, photogenerated electrons from TiO₂ are captured by GO. These electrons are initially used in GO’s reduction, while additional electron transfer results in storage across its sp² network. In the presence of silver ions, deposition of silver nanoparticles (NPs) is accomplished on the GO surface opposite the TiO₂, thus confirming the ability of GO to transport electrons through its plane. Illumination-controlled reduction of silver ions allows for simple selection of particle size and loading, making these semiconductor–graphene–metal (SGM) films ideal for custom catalysis and sensor applications. Initial testing of SGM films as surface-enhanced resonance Raman (SERRS) sensors produced significant target molecule signal enhancements, enabling detection of nanomolar concentrations

<i>Sense and Shoot</i>: Simultaneous Detection and Degradation of Low-Level Contaminants Using Graphene-Based Smart Material Assembly

<i>Smart material</i> nanoassemblies that can simultaneously <i>sense and shoot</i> low-level contaminants from air and water are important for overcoming the threat of hazardous chemicals. Graphene oxide (GO) sheets deposited on mesoscopic TiO₂ films that underpin the deposition of Ag nanoparticles with UV irradiation provide the foundation for the design of a smart material. The Ag particle size is readily controlled through precursor concentration and UV irradiation time. These semiconductor–graphene oxide–metal (SGM) films are SERS-active and hence capable of sensing aromatic contaminants such as 4-nitro­benzene­thiol (4-NBT) in nanomolar range. Increased local concentration of organic molecules achieved through interaction with 2-D carbon support (GO) facilitates low-level detection of contaminants. Upon UV irradiation of 4-NBT-loaded SGM film, one can induce photocatalytic transformations. Thus, each component of the SGM film plays a pivotal role in aiding the detection and degradation of a contaminant dispersed in aqueous solutions. The advantage of using SGM films as multipurpose “detect and destroy” systems for nitro­aromatic molecules is discussed

Down to the Crust: Chemical and Mineralogical Analysis of Ceramic Surface Encrustations on Bronze Age Ceramics from Békés 103, Eastern Hungary

Békés 103, a primarily Middle Bronze Age (c. 1600–1280 calBC) cemetery and settlement on the Great Hungarian Plain, has been investigated by the BAKOTA project since 2011. Ceramics from the site are covered in dense white concretions, and it has been noted during compositional analyses that these vessels exhibit elevated concentrations of several potentially mobile elements in comparison to vessels from regional tell sites. Here, we use a multimethod (optical mineralogy, FT-IR, XRD, XPS, PXRF, SEM-EDS, and LA-ICP-MS) mineralogical and chemical approach to characterize the composition of surface encrustations on ceramics samples from Békés 103. We also chemically map interior paste composition using LA-ICP-MS to identify potential leaching of mobile elements into or out of vessel bodies. We demonstrate that the surface encrustations are primarily composed of calcite but also contain a variety of other mineral and organic constituents indicative of deposition of soil carbonates, phosphates, nitrates, and other inorganic and organic components. We further document the leaching of several mobile elements into ceramic pastes as well as formation of secondary calcite along void, pore, and temper boundaries. The presence of cremated bone and possibly bone ash in close vicinity to many of the studied vessels may also have contributed to the observed patterns of diagenesis. It is likely that similar post-burial processes might affect ceramics from other sites located in low-lying, seasonally inundated contexts

Down to the Crust: Chemical and Mineralogical Analysis of Ceramic Surface Encrustations on Bronze Age Ceramics from Békés 103, Eastern Hungary

Békés 103, a primarily Middle Bronze Age (c. 1600–1280 calBC) cemetery and settlement on the Great Hungarian Plain, has been investigated by the BAKOTA project since 2011. Ceramics from the site are covered in dense white concretions, and it has been noted during compositional analyses that these vessels exhibit elevated concentrations of several potentially mobile elements in comparison to vessels from regional tell sites. Here, we use a multimethod (optical mineralogy, FT-IR, XRD, XPS, PXRF, SEM-EDS, and LA-ICP-MS) mineralogical and chemical approach to characterize the composition of surface encrustations on ceramics samples from Békés 103. We also chemically map interior paste composition using LA-ICP-MS to identify potential leaching of mobile elements into or out of vessel bodies. We demonstrate that the surface encrustations are primarily composed of calcite but also contain a variety of other mineral and organic constituents indicative of deposition of soil carbonates, phosphates, nitrates, and other inorganic and organic components. We further document the leaching of several mobile elements into ceramic pastes as well as formation of secondary calcite along void, pore, and temper boundaries. The presence of cremated bone and possibly bone ash in close vicinity to many of the studied vessels may also have contributed to the observed patterns of diagenesis. It is likely that similar post-burial processes might affect ceramics from other sites located in low-lying, seasonally inundated contexts

To What Extent Do Graphene Scaffolds Improve the Photovoltaic and Photocatalytic Response of TiO₂ Nanostructured Films?

Graphene−TiO₂ nanocomposites synthesized via a solution-based method involving photocatalytic reduction of graphene oxide have been employed as photoanodes. Nearly 90% enhancement in the photocurrent is seen as reduced graphene oxide serves as electron collector and transporter. Additionally, the graphene−TiO₂ nanocomposite electrodes exhibit significant activity for the complete photocatalytic decomposition of 2,4-dichlorophenoxyacetic acid (2,4-D). Combined with safe, solution-based synthetic practices, the promising photocurrent and photocatalytic degradation rates provide the framework and motivation for the implementation of graphene−TiO₂ nanocomposites on larger scales

Sonolytic Design of Graphene−Au Nanocomposites. Simultaneous and Sequential Reduction of Graphene Oxide and Au(III)

High-frequency ultrasound at 211 kHz is effective in developing graphene-based nanoarchitectures. Both simultaneous and sequential reduction steps have been employed to reduce the graphene oxide (GO) and a gold precursor, HAuCl₄. Characterization of the composites by transmission electron microscopy following the reduction process revealed well-dispersed Au nanoparticles on the reduced GO (RGO) sheets that are no more than a few layers thick (1−4 layers). The Raman spectra of the RGO−Au composites showed a distinct surface enhancement of the graphene Raman bands upon increasing the surface coverage of gold nanoparticles. The merits of sonolytic reduction in developing graphene-based composites are discussed

Engineered Substrates Reveal Species-Specific Inorganic Cues for Coral Larval Settlement

The widespread loss of stony reef-building coral populations has been compounded by pervasive recruitment failure, i.e., the low or absent settlement and survival of coral juveniles. To combat global coral reef stressors and rebuild coral communities, restoration practitioners have developed workflows to rear and settle vulnerable coral larvae in the laboratory and subsequently outplant settled juveniles back to natural and artificial reefs. These workflows often make use of the natural biochemical settlement cues present in crustose coralline algae (CCA), which can be presented to swimming larvae as extracts, fragments, or live algal sheets to induce settlement. In this work, we investigated the potential for inorganic chemical cues to complement these known biochemical effects. We designed settlement substrates made from lime mortar (CaCO3) and varied their composition with the use of synthetic and mineral additives, including sands, glasses, and alkaline earth carbonates. In experiments with larvae of two Caribbean coral species, Acropora palmata (elkhorn coral) and Diploria labyrinthiformis (grooved brain coral), we saw additive-specific settlement preferences (>10-fold settlement increase) in the absence of any external biochemical cues. Interestingly, these settlement trends were independent of bulk surface properties such as surface roughness and wettability. Instead, our results suggest that not only can settling coral larvae sense and positively respond to soluble inorganic materials, but that they can also detect localized topographical features more than an order of magnitude smaller than their body width. Our findings open a new area of research in coral reef restoration, in which engineered substrates can be designed with a combination of organic and inorganic additives to increase larval settlement, and perhaps also improve post-settlement growth, mineralization, and defense