Rajesh Kodiyath

Competitive Adsorption of Dopamine and Rhodamine 6G on the Surface of Graphene Oxide

Competitive adsorption-desorption behavior of popular fluorescent labeling and bioanalyte molecules, Rhodamine 6G (R6G) and dopamine (DA), on a chemically heterogeneous graphene oxide (GO) surface is discussed in this study. Individually, R6G and DA compounds were found to adsorb rapidly on the surface of graphene oxide as they followed the traditional Langmuir adsorption behavior. FTIR analysis suggested that both R6G and DA molecules predominantly adsorb on the hydrophilic oxidized regions of the GO surface. Thus, when R6G and DA compounds were adsorbed from mixed solution, competitive adsorption was observed around the oxygen-containing groups of GO sheets, which resulted in partial desorption of R6G molecules from the surface of GO into the solution. The desorbed R6G molecules can be monitored by fluorescence change in solution and was dependent on the DA concentration. We suggest that the efficient competitive adsorption of different strongly bound bioanalytes onto GO-dye complex can be used for the development of sensitive and selective colorimetric biosensors.

Photocatalytic Water Splitting under Visible Light by Mixed-Valence Sn3O4

A mixed-valence tin oxide, (Sn(2+))2(Sn(4+))O4, was synthesized via a hydrothermal route. The Sn3O4 material consisted of highly crystalline {110} flexes. The Sn3O4 material, when pure platinum (Pt) was used as a co-catalyst, significantly catalyzed water-splitting in aqueous solution under illumination of visible light (λ > 400 nm), whereas neither Sn(2+)O nor Sn(4+)O2 was active toward the reaction. Theoretical calculations have demonstrated that the co-existence of Sn(2+) and Sn(4+) in Sn3O4 leads to a desirable band structure for photocatalytic hydrogen evolution from water solution. Sn3O4 has great potential as an abundant, cheap, and environmentally benign solar-energy conversion catalyst.

Assemblies of silver nanocubes for highly sensitive SERS chemical vapor detection

We suggest that silver nanocube (AgNC) aggregates within cylindrical pores (PAM–AgNC) can be employed as efficient nanostructures for highly efficient, robust, tunable, and reusable surface-enhanced Raman scattering (SERS) substrates for trace level organic vapor detection which is a challenging task in chemical detection. We demonstrate the ability to tune both the detection limit and the onset of signal saturation of the substrate by switching the adsorption behavior of AgNCs between highly aggregated and more disperse by varying the number of adsorption-mediating polyelectrolyte bilayers on the pore walls of the membrane. The different AgNC distributions show large differences in the trace vapor detection limit of the common Raman marker benzenethiol (BT) and a widely used explosive binder N-methyl-4-nitroaniline (MNA), demonstrating the importance of the large electromagnetic field enhancement associated with AgNC coupling. The SERS substrate with highly aggregated AgNCs within the cylindrical pores allows for consistent trace detection of mid ppb (∼500) for BT analyte, and a record limit of detection of low ppb (∼3) for MNA vapors with an estimated achievable limit of detection of approximately 600 ppt. The dispersed AgNC aggregates do not saturate at higher ppb concentrations, providing an avenue to distinguish between higher ppb concentrations and increase the effective range of the SERS substrate design. A comparison between the AgNC substrate and an electroless deposition substrate with silver quasi-nanospheres (PAM–AgNS) also demonstrates a higher SERS activity, and better detection limit, by the nanocube aggregates. This is supported by FDTD electromagnetic simulations that suggest that the higher integrated electromagnetic field intensity of the hot spots and the large specific interfacial areas impart greatly improved SERS for the AgNCs. Moreover, we demonstrated that the AgNC substrate can be reused multiple times without significant loss of SERS activity which opens up new avenues for in-field monitoring.

Gold photosensitized SrTiO3 for visible-light water oxidation induced by Au interband transitions

Gold nanoparticle (NP) photosensitization over semiconductors with a large band gap has emerged as a promising strategy for developing visible-light responsive photocatalytic materials. However, its application in harsh photocatalytic oxidation still remains a significant challenge. Furthermore, energetic charge carriers created in Au interband transitions under visible light are frequently ignored in this field. In the current work, for the first time, a remarkable visible-light photocatalytic water oxidation activity (14.9 μmol h−1: 0.2 g catalyst, 5 mmol AgNO3), even slightly higher than that of commercial WO3, was achieved over Au photosensitized SrTiO3 (1.1 wt%). In an elaborate study, electron transfer from gold to SrTiO3 was confirmed by STEM-EDS characterization on selective Ag deposition over SrTiO3. A combined investigation of apparent quantum efficiency results, theoretical simulation study on Au NPs optical excitation and relative band position analysis in Au/SrTiO3 reveals that these hot electrons transferred from gold to SrTiO3 mainly come from Au interband transitions other than plasmon resonance, while leaving holes on Au with enough oxidative potentials are responsible for water oxidation. The capability of involving Au interband transition in photosensitization for visible light water oxidation opens up new opportunities in designing and preparing visible-light responsive photocatalysts.

SERS Effects in Silver‐Decorated Cylindrical Nanopores

Optimization of pore diameter, the placement of nanoparticles, and the transmission of surface-enhanced Raman scattering (SERS) substrates are found to be very critical for achieving high SERS activity in porous alumina-membrane-based substrates. SERS substrates with a pore diameter of 355 nm incorporating silver nanoparticles show very high SERS activity with enhancement factors of 10(10) .

Promoted C–C bond cleavage over intermetallic TaPt3 catalyst toward low-temperature energy extraction from ethanol

Novel intermetallic TaPt3 nanoparticles (NPs) are materialized, which exhibit much higher catalytic performance than state-of-the-art Pt3Sn NPs for electrooxidation of ethanol. In situ infrared-reflection-absorption spectroscopy (IRRAS) elucidates that the TaPt3 NPs cleave the C–C bond in ethanol at lower potentials than Pt NPs, efficiently promoting complete conversion of ethanol to CO2. Single-cell tests demonstrate the feasibility of the TaPt3 NPs as a practical energy-extraction catalyst for ethanol fuels, with more than two times higher output currents than Pt-based cells at high discharge currents.

Selective electro- or photo-reduction of carbon dioxide to formic acid using a Cu–Zn alloy catalyst

A copper-and-zinc (Cu–Zn) alloy material was synthesized using a vacuum sealing method, in which evaporated zinc was reacted with copper film or nanoparticles to form a homogeneous Cu–Zn alloy. This alloy was evaluated as an electrocatalyst and/or cocatalyst for photocatalysis to selectively reduce carbon dioxide to formic acid. Based on the optimised alloy composition, the Cu5Zn8 catalyst exhibited efficient electrochemical CO2 reduction. Furthermore, we constructed a photoelectrochemical (PEC) three-electrode system, in which the Cu5Zn8 film functioned as the cathode for CO2 reduction in the dark and strontium titanate (SrTiO3) served as the photoanode for water oxidation. The PEC system also selectively reduced CO2 to formic acid with a faradaic efficiency of 79.11% under UV-light and the absence of an applied bias potential. SrTiO3 particles decorated with nanoparticles of the Cu–Zn alloy also photocatalytically reduced CO2 to formic acid under UV-light. Isotope trace analysis demonstrated that water served as the electron donor to produce oxygen and organic molecules under UV light, similar to photosynthesis in plants. The Cu–Zn alloy material developed in the present study is composed of ubiquitous and safe materials, and can catalyse CO2 conversion by means of various kinds of renewable energies.

Stimulation of Electro-oxidation Catalysis by Bulk-Structural Transformation in Intermetallic ZrPt3 Nanoparticles

Although compositional tuning of metal nanoparticles (NPs) has been extensively investigated, possible control of the catalytic performance through bulk-structure tuning is surprisingly overlooked. Here we report that the bulk structure of intermetallic ZrPt3 NPs can be engineered by controlled annealing and their catalytic performance is significantly enhanced as the result of bulk-structural transformation. Chemical reduction of organometallic precursors yielded the desired ZrPt3 NPs with a cubic FCC-type structure (c-ZrPt3 NPs). The c-ZrPt3 NPs were then transformed to a different phase of ZrPt3 with a hexagonal structure (h-ZrPt3 NPs) by annealing at temperatures between 900 and 1000 °C. The h-ZrPt3 NPs exhibited higher catalytic activity and long-term stability than either the c-ZrPt3 NPs or commercial Pt/C NPs toward the electro-oxidation of ethanol. Theoretical calculations have elucidated that the enhanced activity of the h-ZrPt3 NPs is attributed to the increased surface energy, whereas the stability of the catalyst is retained by the lowered bulk-free-energy.

CO tolerance of Pt/FeOx catalyst in both thermal catalytic H2 oxidation and electrochemical CO oxidation: the effect of Pt deficit electron state

CO poisoning of Pt catalysts is one of the major challenges to the commercialization of proton exchange membrane fuel cells. One promising solution is to develop CO-tolerant Pt-based catalysts. A facilely synthesized Pt/FeOx catalyst exhibited outstanding CO tolerance in the oxidation of H2 and electrochemical CO stripping. Light-off temperature of H2O formation over Pt/FeOx was achieved even below 30 °C in the presence of 3000 ppm CO at a space velocity of 18 000 mL g-1cat h-1. For the electrochemical oxidation of CO, the onset and peak potentials decreased by 0.17 V and 0.10 V, respectively, in comparison with those of commercial Pt/C. More importantly, by a combination of hard X-ray photoemission spectroscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies it was found that the decreased electron density of Pt in Pt/FeOx enhanced the mobility of adsorbed CO, suppressed Pt-CO bonding and significantly increased the CO tolerance of Pt/FeOx.

Mixed-valence NaSb3O7 support toward improved electrocatalytic performance in the oxygen-reduction reaction

Nanocrystals of sodium antimony oxide, NaSb3O7 (pyrochlore structure, a = 1.030 nm), act as an efficient catalyst support for the electrocatalytic oxygen-reduction reaction (ORR) in acidic media. The NaSb3O7 nanocrystals (edge length ∼ 150 nm) were synthesized by hydrothermal decomposition of SbCl5 in aqueous solution of NaOH. The NaSb3O7 nanocrystals were then decorated with Pt nanoparticles by chemical reduction of H2PtCl6 in water to yield an ORR catalyst, Pt/NaSb3O7. The Pt/NaSb3O7 exhibited higher ORR performance than the state-of-the-art Pt/TiO2- or Pt/C catalysts in terms of the +40 mV higher half-wave reduction potential and the retained electrochemical surface area than the Pt/TiO2 after 10 000-times repeated ORR in an acidic electrolyte. Unlike NaSb3O7, Pt-decorated Sb2O5 (Pt/Sb2O5) was much less active than the Pt/TiO2 or Pt/C. The enhanced ORR activity of the Pt/NaSb3O7 may be attributed to the promoted electron hopping between the Sb3+ and Sb5+ ions in mixed-valence Na1+(Sb3+Sb25+)O7, which is inhibited in single-valence Sb25+O5.