Rini Ravindranath

Highly Stretchable and Sensitive Photodetectors Based on Hybrid Graphene and Graphene Quantum Dots

Stretchable devices possess great potential in a wide range of applications, such as biomedical and wearable gadgets and smart skin, which can be integrated with the human body. Because of their excellent flexibility, two-dimensional (2D) materials are expected to play an important role in the fabrication of stretchable devices. However, only a limited number of reports have been devoted to investigating stretchable devices based on 2D materials, and the stretchabilities were restricted in a very small strain. Moreover, there is no report related to the stretchable photodetectors derived from 2D materials. Herein, we demonstrate a highly stretchable and sensitive photodetector based on hybrid graphene and graphene quantum dots (GQDs). A unique rippled structure of poly(dimethylsiloxane) is used to support the graphene layer, which can be stretched under an external strain far beyond published reports. The ripple of the device can overcome the native stretchability limit of graphene and enhance the carrier generation in GQDs due to multiple reflections of photons between the ripples. Our strategy presented here can be extended to many other material systems, including other 2D materials. It therefore paves a key step for the development of stretchable electronics and optical devices.

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+.

Wrinkled 2D Materials: A Versatile Platform for Low-Threshold Stretchable Random Lasers

A stretchable, flexible, and bendable random laser system capable of lasing in a wide range of spectrum will have many potential applications in next- generation technologies, such as visible-spectrum communication, superbright solid-state lighting, biomedical studies, fluorescence, etc. However, producing an appropriate cavity for such a wide spectral range remains a challenge owing to the rigidity of the resonator for the generation of coherent loops. 2D materials with wrinkled structures exhibit superior advantages of high stretchability and a suitable matrix for photon trapping in between the hill and valley geometries compared to their flat counterparts. Here, the intriguing functionalities of wrinkled reduced graphene oxide, single-layer graphene, and few-layer hexagonal boron nitride, respectively, are utilized to design highly stretchable and wearable random laser devices with ultralow threshold. Using methyl-ammonium lead bromide perovskite nanocrystals (PNC) to illustrate the working principle, the lasing threshold is found to be ≈10 µJ cm-2 , about two times less than the lowest value ever reported. In addition to PNC, it is demonstrated that the output lasing wavelength can be tuned using different active materials such as semiconductor quantum dots. Thus, this study is very useful for the future development of high-performance wearable optoelectronic devices.

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.

Metal-deposited bismuth oxyiodide nanonetworks with tunable enzyme-like activity: sensing of mercury and lead ions

In this study, we demonstrate that the enzyme-like activity of bismuth oxyiodide (BiOI) nanonetworks can be regulated through homogeneous deposition of metal atoms/ions or nanoparticles. Bismuth oxyhalide (BiOX; X = Cl, Br or I) nanostructures were prepared from a simple mixture of bismuth ions (Bi3+) and halide ions (X−) in aqueous solution. The BiOI nanonetworks exhibited much stronger (>25-fold) peroxidase-like activity than BiOCl or BiOBr nanosheets. In situ formation and deposition of gold nanoparticles (Au NPs) onto BiOI nanonetworks greatly enhanced the oxidase-like activity of the nanocomposites. The deposition of Ni, Zn or Mn on the BiOI nanonetworks boosted their peroxidase-like activity by at least 3-fold. Moreover, the catalase-like activity of the BiOI nanonetworks was elevated after deposition of MnO2 or ZnO nanoparticles. The enzyme-like activity of BiOI regulated by the deposition of metals was mainly due to the changes in the electronic and band structures of the BiOX nanonetworks, and the existence of surface metal atoms/ions in various oxidation states. We used the Au NPs/BiOI nanocomposites and NiO NPs/BiOI nanocomposites for the detection of Hg2+ and Pb2+ heavy metal ions, respectively, based on the suppression of the enzyme-like activity of the nanocomposite after deposition of these metal ions. These BiOI nanocomposite-based probes allow the selective detection of Hg2+ and Pb2+ down to nanomolar quantities. The practicality of these two nanozyme probes was validated by analysis of Hg2+ and Pb2+ ions in environmental water samples (tap water, river water, lake water, and sea water).

Green synthesis of Si–GQD nanocomposites as cost-effective catalysts for oxygen reduction reaction

Hybrid silicon nanosheets (NSs)–graphene quantum dot nanocomposites (Si–GQD NCs) were prepared from a mixture of GQDs and Si NSs in ethanol at 25 °C for 2 h and used as a catalyst for oxygen reduction reactions (ORR) in direct methanol fuel cells (DMFCs). GQDs were prepared from fenugreek seed extracts (300 °C, 8 h) and wrinkled Si NSs were obtained from pyrolysis of rice husks (700 °C, 2 h). The Si–GQD NCs fabricated glassy carbon electrode (GCE) has greater electrocatalytic activity for ORR in comparison to Si NSs and GQDs modified GCEs, showing the synergistic effect provided by Si NSs and GQDs. The GQDs enhance O2 adsorption and ORR activity, while Si NSs function as a support to increase charge transfer. Additionally, high surface area and the wrinkled structure of Si NSs allow efficient mass transfer, leading to greater ORR activity. The onset potential of the Si–GQD NC electrode is −0.33 V (versus Ag/AgCl) with a current density of 2.61 ± 0.27 mA cm−2, showing greater electrocatalytic activity. Furthermore, the Si–GQD NC electrode exhibits greater tolerance against methanol and carbon monoxide poisoning than the Pt/C electrode. The environmentally-friendly, active, stable and inexpensive Si–GQD NCs hold great potential for DMFCs.

Carbon–boron core–shell microspheres for the oxygen reduction reaction

Core–shell carbon nanomaterials (CNMs) with high oxygen reduction reaction (ORR) efficiency and greater stability have become potential alternatives to the expensive Pt/C used in direct methanol fuel cells (DMFCs). Here, we demonstrate a simple strategy for the preparation of electroactive metal-free catalysts from wastes (red onion skins) for improving ORR efficiency. Through hydrogen bonding and π–π stacking interactions between/among aromatic rings of the molecules, many CNMs with sizes mostly smaller than 20 nm are self-assembled on the surface of each carbon microsphere having an average diameter of 0.9 ± 0.1 μm. To further improve the electrocatalytic activity of CNMs@C microspheres, they have been subjected to treatment with boric acid, leading to the formation of a boron (B) shell on each CNMs@C microsphere. The B0.5CNMs@C1.0 electrode exhibits higher catalytic activity than the CNMs@C electrode, with advantages of high stability and tolerance against methanol and CO poisoning.

Synthesis, Optical Properties, and Sensing Applications of Gold Nanodots

In this Personal Account, we briefly address our journey in developing photoluminescent nanomaterials for sensing purposes, with a focus on gold nanodots (Au NDs). Their synthetic strategies, optical properties, and sensing applications are emphasized. The Au NDs can be simply prepared from the etching of small-sized Au nanoparticles (<3 nm in diameter) by thiol compounds such as 11-mercaptoundecanoic acid under alkaline conditions. This simple approach allows the preparation of various functional Au NDs by choosing different thiol compounds as etching agents. Since the optical properties of Au NDs are highly dependent on the core and shell of each Au ND, the selection of etching reagents is important. Over the years we have developed various sensing systems using Au NDs for the detection of metal ions, anions, and proteins, based on analyte-induced photoluminescence quenching/enhancement of Au NDs as a result of changes in their oxidation state, shell composition, and structure.