Prathik Roy

Quantum dot-sensitized solar cells incorporating nanomaterials

Quantum dot-sensitized solar cells (QDSSCs) are interesting energy devices because of their (i) impressive ability to harvest sunlight and generate multiple electron/hole pairs, (ii) ease of fabrication, and (iii) low cost. The power conversion efficiencies (η) of most QDSSCs (typically <4%) are, however, less than those (up to 12%) of dye-sensitized solar cells, mainly because of narrow absorption ranges and charge recombination occurring at the QD-electrolyte and TiO(2)-electrolyte interfaces. To further increase the values of η of QDSSCs, it will be necessary to develop new types of working electrodes, sensitizers, counter electrodes and electrolytes. This Feature Article describes the nanomaterials that have been used recently as electronic conductors, sensitizers and counter electrodes in QDSSCs. The nature, size, morphology and quantity of these nanomaterials all play important roles affecting the efficiencies of electron injection and light harvesting. We discuss the behavior of several important types of semiconductor nanomaterials (sensitizers, including CdS, Ag(2)S, CdSe, CdTe, CdHgTe, InAs and PbS) and nanomaterials (notably TiO(2), ZnO and carbon-based species) that have been developed to improve the electron transport efficiency of QDSSCs. We point out the preparation of new generations of nanomaterials for QDSSCs and the types of electrolytes, particularly iodide/triiodide electrolytes (I(-)/I(3)(-)), polysulfide electrolytes (S(2-)/S(x)(2-)), and cobalt redox couples ([Co(o-phen)(3)(2+)/(3+)]), that improve their lifetimes. With advances in nanotechnology, we foresee significant improvements in the efficiency (η > 6%) and durability (>3000 h) of QDSSCs.

Synthesis of graphene-ZnO-Au nanocomposites for efficient photocatalytic reduction of nitrobenzene.

A simple hydrothermal method of preparing highly photocatalytic graphene-ZnO-Au nanocomposites (G-ZnO-Au NCs) has been developed. Zinc acetate and graphene oxide are reduced by catechin to form graphene-zinc oxide nanospheres (G-ZnO NSs; average diameter of (45.3 ± 3.7) nm) in the presence of ethylenediamine (EDA) as a stabilizing agent and gold nanorods (Au NRs) at 300 °C for 2 h. Then Au NRs are deposited onto as-formed G-ZnO NSs to form G-ZnO-Au NCs. Upon ultraviolet light activation, G-ZnO-Au NCs (4 mg mL(-1)) in methanol generates electron-hole pairs. Methanol (hydroxyl group) assists in trapping holes, enabling photogenerated electrons to catalyze reduction of nitrobenzene (NB) to aniline with a yield of 97.8% during a reaction course of 140 min. The efficiency of G-ZnO-Au NCs is 3.5- and 4.5-fold higher than those provided by commercial TiO2 and ZnO NSs, respectively. Surface assisted laser desorption/ionization mass spectrometry has been for the first time applied to detect the intermediates (nitrosobenzene and phenylhydroxylamine) and major product (aniline) of NB through photoelectrocatalytic or photocatalytic reactions. The result reveals that the reduction of NB to aniline is through nitrosobenzene to phenylhydroxylamine in the photoelectrocatalytic reaction, while via nitrosobenzene directly in the photocatalytic reaction. G-ZnO-Au NC photocatalyst holds great potential in removal of organic pollutants like NB and in the production of aniline.

Plant leaf-derived graphene quantum dots and applications for white LEDs

Graphene quantum dots (GQDs) have been prepared for the first time using raw plant leaf extracts of Neem (Azadirachta indica) and Fenugreek (Trigonella foenum-graecum) by a facile, hydrothermal method at 300 °C for 8 hours in water, without the need of any passivizing, reducing agents or organic solvents. High resolution transmission electron microscope studies showed that the average sizes of the GQDs from Neem (N-GQDs) and Fenugreek (F-GQDs) were 5 and 7 nm respectively. N-GQDs and F-GQDs exhibit high quantum yields of 41.2% and 38.9% respectively. Moreover, the GQDs were utilized to prepare a white light converting cap based on the red-green-blue (RGB) color mixing method.

Synthesis of photoluminescent carbon dots for the detection of cobalt ions

We have developed a simple assay for the sensing of cobalt ions (Co2+), based on the analyte induced photoluminescence (PL) quenching of carbon dots (C-dots). The C-dots (mean diameter 3.6 ± 0.3 nm) prepared from L-cysteine through a simple hydrothermal process at 300 °C for 2 h have a quantum yield of 13.2%. The C-dots have strong blue PL with a maximum PL intensity at 395 nm under an excitation wavelength of 325 nm. Through the reactions of Co2+ ions with cysteine molecules/residues on the surfaces of the C-dots, non-photoluminescent CoxSy nanoparticles are formed. As-formed CoxSy nanoparticles and the C-dots further form aggregates in the solution, leading to PL quenching. The C-dot probe allows detection of Co2+ ions over a concentration range from 10 nM to 100 μM (R2 = 0.992). This reliable, rapid, sensitive, and selective C-dot probe has been utilized for the determination of the concentrations of Co2+ ions in vitamin B12 and natural water samples.

Preparation of Photocatalytic Au–Ag2Te Nanomaterials

A facile approach has been developed for the preparation of various morphologies of Au-Ag(2)Te nanomaterials (NMs) that exhibit strong photocatalytic activity. Te NMs (nanowires, nanopencils, and nanorice) were prepared from TeO(2) in the presence of various concentrations (16, 8, and 4 M) of a reducing agent (N(2)H(4)) at different temperatures (25 and 60 °C). These three Te NMs were then used to prepare Au-Ag(2)Te NMs by spontaneous redox reactions with Au(3+) and Ag(+) ions sequentially. The Au-Ag(2)Te nanopencils exhibit the highest activity toward degradation of methylene blue and formation of active hydroxyl radicals on solar irradiation, mainly because they absorb light in the visible region most strongly. All three differently shaped Au-Ag(2)Te NMs (10 μg mL(-1)) provide a death rate of Escherichia coli greater than 80% within 60 min, which is higher than that of 51% for commercial TiO(2) nanoparticles (100 μg mL(-1)). Under light irradiation, the Au NPs in Au-Ag(2)Te NMs enhance the overall photo-oxidation ability of Ag(2)Te NMs through faster charge separation because of good contact between Ag(2)Te and Au segments. With high antibacterial activity and low toxicity toward normal cells, the Au-Ag(2)Te NMs hold great potential for use as efficient antibacterial agents.

Synthesis and antimicrobial activity of gold/silver-tellurium nanostructures.

Gold-tellurium nanostructures (Au-Te NSs), silver-tellurium nanostructures (Ag-Te NSs), and gold/silver-tellurium nanostructures (Au/Ag-Te NSs) have been prepared through galvanic reactions of tellurium nanotubes (Te NTs) with Au(3+), Ag(+), and both ions, respectively. Unlike the use of less environmentally friendly hydrazine, fructose as a reducing agent has been used to prepare Te NTs from TeO2 powders under alkaline conditions. The Au/Ag-Te NSs have highly catlaytic activity to convert nonfluorescent Amplex Red to form fluorescent product, revealing their great strength of generating reactive oxygen species (ROS). Au/Ag-Te NSs relative to the other two NSs exhibit greater antimicrobial activity toward the growth of E. coli, S. enteritidis, and S. aureus; the minimal inhibitory concentration (MIC) values of Au/Ag-Te NSs were much lower (>10-fold) than that of Ag-Te NSs and Au-Te NSs. The antibacterial activity of Au/Ag-Te NSs is mainly due to the release of Ag(+) ions and Te-related ions and also may be due to the generated ROS which destroys the bacteria membrane. In vitro cytotoxicity and hemolysis analyses have revealed their low toxicity in selected human cell lines and insignificant hemolysis in red blood cells. In addition, inhibition zone measurements using a Au/Ag-Te NSs-loaded konjac jelly film have suggested that it has great potential in practial application such as wound dressing for reducing bacterial wound infection. Having great antibacterial activitiy and excellent biocompatibility, the low-cost Au/Ag-Te NSs hold great potential as effective antimicrobial drugs.

Polymer/reduced graphene oxide functionalized sponges as superabsorbents for oil removal and recovery.

Polyurethane dish-washing (PU-DW) sponges are functionalized sequentially with polyethylenimine (PEI) and graphene oxide (GO) to form PEI/reduced graphene oxide (RGO) PU-DW sponges. The PEI/RGO PU-DW sponge consists of PEI/RGO sheets having numerous pores, with diameters ranging from 236 to 254nm. To further enhance hydrophobicity and absorption capacity of oil, PEI/RGO PU-DW sponge is further coated with 20% phenyltrimethoxysilane (PTMOS). The PTMOS/PEI/RGO PU-DW sponge absorbs various oils within 20s, with maximum absorption capacity values of 880% and 840% for bicycle chain oil and motorcycle engine oil, respectively. The absorbed oils were released completely by squeezing or immersed in hexane. The PTMOS/PEI/RGO PU-DW sponge efficiently separates oil/water mixtures through a flowing system. Having the advantages of faster absorption rate, reusability, and low cost, the PTMOS/PEI/RGO PU-DW sponge holds great potential as a superabsorbent for efficient removal and recovery of oil spills as well as for the separation of oil/water mixtures.

Effects of deposited ions on the photocatalytic activity of TiO2–Au nanospheres

Photocatalytic TiO2–Au nanospheres (TiO2–Au NSs, 206 ± 23.7 nm) have been prepared and used as catalysts for the photo degradation of methylene blue (MB) and for the reduction of Cr6+ to Cr3+. TiO2 NSs are firstly prepared from titanium isopropoxide (TIP) via a solvothermal method. The TiO2 NSs are then sequentially modified with poly-(sodium-4-styreneulfonate) (PSS) and poly(diallyldimethylammonium chloride) (PDADMAC), which then interact with Au NPs (15 ± 1.3 nm) as seeds. Through reduction of HAuCl4 by ascorbic acid, core–shell structures of TiO2–Au NSs are prepared. Under UV irradiation, TiO2–Au NSs provide highly catalytic activity for the degradation of MB and reduction of Cr6+ within 15 and 60 min, respectively. The TiO2–Au NSs relative to commercial TiO2 (P25) and TiO2 NSs provide 1.8 and 1.2-fold activity higher for the photo degradation of MB, and 4.3 and 1.8-fold higher for the reduction of Cr6+. TiO2–Au/Hg and TiO2–Au/Ag NSs that are prepared from the deposition of Hg2+ and Ag+ onto TiO2–Au NSs, respectively, allow degradation of MB within 10 min, with activities 4.2- and 3.3-fold greater than that of the TiO2–Au NSs. The present study reveals that TiO2–Au/Hg and TiO2–Au/Ag NSs are effective for removal of organic pollutants, while TiO2–Au NSs are useful for the reduction of Cr6+.

Dirac point induced ultralow-threshold laser and giant optoelectronic quantum oscillations in graphene-based heterojunctions

The occurrence of zero effective mass of electrons at the vicinity of the Dirac point is expected to create new paradigms for scientific research and technological applications, but the related discoveries are rather limited. Here, we demonstrate that a simple architecture composed of graphene quantum dots sandwiched by graphene layers can exhibit several intriguing features, including the Dirac point induced ultralow-threshold laser, giant peak-to-valley ratio (PVR) with ultra-narrow spectra of negative differential resistance and quantum oscillations of current as well as light emission intensity. In particular, the threshold of only 12.4 nA cm−2 is the lowest value ever reported on electrically driven lasers, and the PVR value of more than 100 also sets the highest record compared with all available reports on graphene-based devices. We show that all these intriguing phenomena can be interpreted based on the unique band structures of graphene quantum dots and graphene as well as resonant quantum tunneling.In graphene, electrons possess zero effective mass in proximity to the Dirac point, an unusual feature that could trigger the development of novel photonic devices. Here, the authors combine graphene quantum dots with two graphene layers and observe laser action with ultralow threshold.

Fe2O3/Al2O3 microboxes for efficient removal of heavy metal ions

Iron oxide/aluminum oxide microboxes (Fe2O3/Al2O3 MBs) with cubic structures (1 ± 0.09 μm) possessing large specific surface area (208.3 m2 g−1) and high adsorption capacity (216 mg g−1) were prepared and utilized for the removal of mercury (Hg2+) (100 ppm) from various samples, including tap water, lake water and tomato juice, with efficiencies of 98.2 ± 0.4, 98.5 ± 0.3 and 97.1 ± 0.5%, respectively. The Fe2O3/Al2O3 MBs are stable, allowing the adsorbed metal species to be removed from their surfaces with 2 M HCl. The Fe2O3/Al2O3 MBs can be reused up to five times after being treated with 2 M HCl. Furthermore, the Fe2O3/Al2O3 MBs are efficient adsorbents for the removal of four metal ions such as Hg2+, cadmium (Cd2+), copper (Cu2+), and lead (Pb2+) ions from soil samples, mainly because of a synergetic effect provided by the two metal oxides and high surface area. This low-cost, effective, and stable Fe2O3/Al2O3 adsorbent holds great potential for the removal of Hg2+ and other heavy metal ions from contaminated sources such as water and soil.