W Doping in Ni12P5 as a Platform to Enhance Overall Electrochemical Water Splitting.

Bifunctional electrocatalysts for efficient hydrogen generation from water splitting must overcome both the sluggish water dissociation step of the alkaline hydrogen evolution half-reaction (HER) and the kinetic barrier of the anodic oxygen evolution half-reaction (OER). Nickel phosphides are a promising catalysts family and are known to develop a thin active layer of oxidized Ni in an alkaline medium. Here, Ni12P5 was recognized as a suitable platform for the electrochemical production of γ-NiOOH─a particularly active phase─because of its matching crystallographic structure. The incorporation of tungsten by doping produces additional surface roughness, increases the electrochemical surface area (ESCA), and reduces the energy barrier for electron-coupled water dissociation (the Volmer step for the formation of Hads). When serving as both the anode and cathode, the 15% W-Ni12P5 catalyst provides an overall water splitting current density of 10 mA cm-2 at a cell voltage of only 1.73 V with good durability, making it a promising bifunctional catalyst for practical water electrolysis.

Structural Transformation of SnS2 to SnS by Mo Doping Produces Electro/Photocatalyst for Hydrogen Production.

SnS and SnS2 are layered semiconductors, with potential promising properties for electro- and photocatalytic hydrogen (H2 ) production. The vast knowledge in preparation and modification of layered structures was still not employed successfully in this system to fully maximize its potential. Here, the first report of structural transformation of SnS2 into SnS with Mo-doping as a bifunctional catalyst for the hydrogen evolution reaction (HER) is reported. The structural phase transition optimized the properties of the material, providing a more delicate morphology with additional catalytic sites. The electrochemical studies showed overpotential of 377 mV at 10 mA cm-2 for HER with Tafel slopes of 100 mV dec-1 in 0.5 m H2 SO4 for 10 % Mo-SnS. The same structure acts as an efficient photocatalyst in the generation of H2 from water under visible illumination with rate of 0.136 mmol g-1 h-1 of H2 , which is 20 times higher than pristine SnS2 under visible light.

Visible transparent white light emitting ink from a Ce3+ sensitized monodispersed Tb,Sm co-doped LaF3@C-dot nanocomposite.

Bright white light emission (CIE values 0.32 and 0.33) has been achieved by a single excitation source (254 nm) from a monodispersed nanocomposite composed of Ce3+, Tb3+ and Sm3+ doped LaF3 nanocrystals (NCs) and N-doped C-dots where Ce3+ acts as an excellent sensitizer. Controlled deposition of the C-dots onto the surface of the LaF3 NCs impedes conventional aggregation induced quenching (AIQ) of the C-dots in solid state. The transparent colloidal ink was coated onto an UV chip to fabricate a white light emitting diode (WLED) with a luminous efficacy of 8.72 lm W-1.

Efficient Charge Separation in Plasmonic ZnS@Sn:ZnO Nanoheterostructure: Nanoscale Kirkendall Effect and Enhanced Photophysical Properties.

Tetravalent Sn doped ZnO nanocrystals show excellent plasmonic absorbance in the visible region. Plasmonic ZnS@Sn:ZnO core-shell heterostructures have been synthesized by the anion exchange process where the O2- is exchanged by S2- anion. An increase of sulfur concentration induces interior hollow structures arising from the different diffusion rates of O2- and S2- ions. Gradual transformation of wurtztie ZnO nanocrystals in the anion exchange process stabilizes the wurtzite crystalline phase of ZnS. Carrier concentration and various types of intrinsic defect states in both ZnO and ZnS result in ultraviolet, blue, and green emissions. The coexistence of exciton-plasmon coupling in the same nanoparticle and efficient electron-hole separation in type II heterostructure increases the photocatalytic activity and photo current gain.

Size specific emission in peptide capped gold quantum clusters with tunable photoswitching behavior.

Three different types of fluorescent gold clusters (namely blue, green and red emitting) have been prepared from a gold precursor (chloroauric acid) under moderate conditions in aqueous medium. A cysteine containing dipeptide has been used for the formation of these quantum clusters as this peptide molecule contains a thiol group in the side chain to cap these nascently formed clusters and the free amino and carboxylic moieties assist in water solubility. Thus, the clusters are also environmentally friendly as the capped peptide is made up of only naturally occurring protein amino acids. These clusters have been well characterized by using UV-visible, fluorescence, X-ray photoelectron spectroscopy (XPS)spectroscopy, matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) and ultrahigh resolution field emission gun-transmission electron microscopy (UHR-FEG-TEM). Arrangements of gold atoms and their interaction with the corresponding ligands in three different fluorescent clusters have been predicted computationally. The excited state behavior of three different clusters has also been studied using time dependent density functional theory (TD-DFT). Time correlated single photon counting (TCSPC) and computational studies suggest intersystem crossing (S1 → T1) in the case of red-emitting Au23 clusters. Interestingly, these gold clusters exhibit semiconducting and photoswitching properties (Ion/Ioff), which are shown to be controlled by varying the size of these clusters. This holds future promise of using these gold cluster based nanomaterials for optoelectronic applications.

Shape Controlled Plasmonic Sn Doped CdO Colloidal Nanocrystals: A Synthetic Route to Maximize the Figure of Merit of Transparent Conducting Oxide.

The synthesis of different anisotropic shaped (eight different shapes) Sn4+ doped CdO (Sn:CdO) colloidal nanocrystals (NCs) by precise tuning of precursor reactivity and proper choice of capping agent is reported. In all these systems, formation of Sn:CdO quantum dots (QDs) of 2-3 nm is identified at very early stage of reaction. The colloidally stable QDs act as a continuous source for the formation of primary nanoparticles that can be transformed selectively into specific type of nanoparticle morphology. The specific facet stabilization of fcc (face centered cubic)CdO is predicted by particular choice of ligand. Fine tuning of plasmonic absorbance band can be achieved by variation of Sn4+ doping concentration. Different anisotropic Sn:CdO NCs exhibit interesting shape dependent plasmonic absorbance features in NIR region. High quality crack free uniform dense thin film has been deposited on glass substrate to make high quality transparent conducting oxide (TCO) coatings. figure of merit of TCO can be maximized as high as 0.523 Ω-1 with conductivity of 43 600 S cm-1 and visible transmittance of ≈85% which is much higher than commercially available tin doped indium oxide and other transparent electrodes.

Enhanced photophysical properties of plasmonic magnetic metal-alloyed semiconductor heterostructure nanocrystals: a case study for the Ag@Ni/Zn1-xMgxO system.

Understanding the effect of homovalent cation alloying in wide band gap ZnO and the formation of metal-semiconductor heterostructures is very important for maximisation of the photophysical properties of ZnO. Nearly monodisperse ZnO nanopyramid and Mg alloyed ZnO nanostructures have been successfully synthesized by one pot decomposition of metal stearate by using oleylamine both as activating and capping agent. The solid solubility of Mg(ii) ions in ZnO is limited to ∼30% without phase segregation. An interesting morphology change is found on increasing Mg alloying: from nanopyramids to self-assembled nanoflowers. The morphology change is explained by the oriented attachment process. The introduction of Mg into the ZnO matrix increases the band gap of the materials and also generates new zinc interstitial (Zni) and oxygen vacancy related defects. Plasmonic magnetic Ag@Ni core-shell (Ag as core and Ni as shell) nanocrystals are used as a seed material to synthesize Ag@Ni/Zn1-xMgxO complex heterostructures. Epitaxial growth is established between Ag(111) and ZnO(110) planes in the heterostructure. An epitaxial metal-semiconductor interface is very crucial for complete electron-hole (e-h) separation and enhancement of the exciton lifetime. The alloyed semiconductor-metal heterostructure is observed to be highly photocatalytically active for dye degradation as well as photodetection. Incorporation of magnetic Ni(0) makes the photocatalyst superparamagnetic at room temperature which is found to be helpful for catalyst regeneration.

Excitation dependent multicolor emission and photoconductivity of Mn, Cu doped In2S3 monodisperse quantum dots.

Indium sulphide (In2S3) quantum dots (QDs) of average size 6 ± 2 nm and hexagonal nanoplatelets of average size 37 ± 4 nm have been synthesized from indium myristate and indium diethyl dithiocarbamate precursors respectively. The absorbance and emission band was tuned with variation of nanocrytal size from very small in the strong confinement regime to very large in the weak confinement regime. The blue emission and its shifting with size has been explained with the donor-acceptor recombination process. The 3d element doping (Mn(2+) and Cu(2+)) is found to be effective for formation of new emission bands at higher wavelengths. The characteristic peaks of Mn(2+) and Cu(2+) and the modification of In(3+) peaks in the x-ray photoelectric spectrum (XPS) confirm the incorporation of Mn(2+) and Cu(2+) into the In2S3 matrix. The simulation of the electron paramagnetic resonance signal indicates the coexistence of isotropic and axial symmetry for In and S vacancies. Moreover, the majority of Mn(2+) ions and sulphur vacancies (VS ) reside on the surface of nanocrystals. The quantum confinement effect leads to an enhancement of band gap up to 3.65 eV in QDs. The formation of Mn 3d levels between conduction band edge and shallow donor states is evidenced from a systematic variation of emission spectra with the excitation wavelength. In2S3 QDs have been established as efficient sensitizers to Mn and Cu emission centers. Fast and slow components of photoluminescence (PL) decay dynamics in Mn and Cu doped QDs are interpreted in terms of surface and bulk recombination processes. Fast and stable photodetctors with high photocurrent gain are fabricated with Mn and Cu doped QDs and are found to be faster than pure In2S3. The fastest response time in Cu doped QDs is an indication of the most suitable system for photodetector devices.

Maximizing the photo catalytic and photo response properties of multimodal plasmonic Ag/WO(3-x) heterostructure nanorods by variation of the Ag size.

High quality nearly monodisperse colloidal WO3-x nanorods with an aspect ratio ∼18 were synthesized using the thermal decomposition technique. The effects of a capping agent and an activating agent on the nanorod aspect ratio have been studied. Excess carrier concentration due to large oxygen vacancy and smaller width of the nanorods compared to the Bohr exciton radius gives rise to an increase of the band gap. Shape anisotropy in nanorods results in two plasmonic absorbance bands at about 890 nm and 5900 nm corresponding to short axis and long axis plasmon modes. The short axis mode reveals an excellent plasmonic sensitivity of ∼345 nm per refractive index. A plasmonic photocatalysis process based on WO3-x nanorods has been developed to synthesize Ag/WO3-x heterostructures consisting of multiple Ag dots with ∼2 nm size, randomly decorated on the surface of the WO3-x nanorods. Long time irradiation leads to an increase in the size (5 nm) of Ag nanocrystals concomitant with decrease in the number of Ag nanocrystals attached per WO3-x nanorod. Plasmonic photocatalysis followed by thermal annealing produces only one Ag nanocrystal of size ∼10 nm on each WO3-x nanorod. Red shifting and broadening of plasmon bands of Ag nanocrystals and WO3-x nanorods confirm the formation of heterostructures between the metal and semiconductor. Detailed transmission electron micrograph analysis indicates the epitaxial growth of Ag nanocrystals onto WO3-x nanorods. A high photocurrent gain of about 4000 is observed for Ag (10 nm)/WO3-x heterostructures. The photodegradation rate for Rhodamine-B and methylene blue is maximum for Ag (10 nm)/WO3-x heterostructures due to efficient electron transfer from WO3-x nanorods to Ag nanocrystals. Metal plasmon-semiconductor exciton coupling, prominent plasmon absorbance of metal nanoparticles, and formation of an epitaxial interface are found to be the important factors to achieve the maximum photocatalytic activity and fabrication of a high speed photodetector device by employing the heterostructures.

Carrier concentration dependent optical and electrical properties of Ga doped ZnO hexagonal nanocrystals.

Colloidal trivalent gallium (Ga) doped zinc oxide (ZnO) hexagonal nanocrystals have been prepared to introduce more carrier concentration into the wide band gap of ZnO. The dopant (Ga) modifies the morphology and size of ZnO nanocrystals. Low content of Ga enhances the optical band gap of ZnO due to excess carrier concentration in the conduction band of ZnO. The interaction among free carriers arising from higher concentration of Ga gives rise to narrowing of the band gap. Surface plasmon resonance absorption appears in the infrared region due to excessive carrier concentration. A broad emission band consists of blue, yellow and green colors associated with different native defects of ZnO. Intrinsic defects and extrinsic dopant Ga control the defect related emission spectrum in the visible region. Replacement of Zn by Ga induces a room temperature metallic state in a degenerate semiconductor. Cationic disorder leads to metal-semiconductor transition at low temperature strongly dependent on the concentration of Ga. Pure semiconducting behavior up to about 80 K is observed for the highest amount of Ga. Temperature dependent metal-semiconductor transition has been interpreted by localization of charge carriers due to disorder arising from random Ga substitution.

Tunable surface plasmon resonance and enhanced electrical conductivity of In doped ZnO colloidal nanocrystals.

We report a new synthesis process of colloidal indium (In) doped zinc oxide (ZIO) nanocrystals by a hot injection technique. By fine tuning the synthesis we reached the same nucleation temperature for indium oxide and zinc oxide which helped us to study a dopant precursor dependent In incorporation into the ZnO matrix by using different In sources. The dopant induced shape evolution changes the hexagonal pyramid structured ZnO to a platelet like structure upon 8% In doping. The introduction of trivalent In(3+) into the ZnO lattice and consequent substitution of divalent Zn(2+) generates free electrons in the conduction band which produces a plasmonic resonance in the infrared region. The electron concentration controls plasmon frequency as well as the band gap of host ZnO. The variation of the band gap and the modification of the conduction band have been explained by the Burstein-Moss effect and Mie's theory respectively. The In dopant changes the defect chemistry of pure ZnO nanocrystals which has been studied by photoluminescence and other spectroscopic measurements. The nanocrystals are highly stable in the organic medium and can be deposited as a crack free thin film on different substrates. Careful ligand exchange and thermal annealing of the spin cast film lead to a good conductive film (720 Ω per square to 120 Ω per square) with stable inherent plasmonic absorption in the infrared and 90% transmittance in the visible region. A temperature induced metal-semiconductor transition was found for doped ZnO nanocrystals. The transition temperature shifts to a lower temperature with increase of the doping concentration.

Effect of oleic acid ligand on photophysical, photoconductive and magnetic properties of monodisperse SnO2 quantum dots.

Oleic acid capped monodisperse SnO(2) quantum dots (QDs) of size 2.7 nm were synthesized by thermal decomposition and oxidation of Sn(II)(oleate) complex in high boiling nonpolar solvent octadecene using oleic acid as a capping agent and N-methylmorpholine N-oxide as an oxidizing agent. FTIR, DSC and TGA were employed to understand the growth of the oleic acid capped SnO(2) QDs through the decomposition of metal fatty acid complex. The surface defect-related luminescence properties of the QDs were demonstrated using steady-state and time-resolved spectroscopy. The oleic acid capping on the QD surface modifies the electronic structure of SnO(2) and generates blue emission. Moreover the surface capping on the QDs diminishes the photocatalytic activity of bare SnO(2) QDs due to absence of surface oxygen and adsorbed hydroxyl group on the surface of the capped QDs. The capping by the long chain ligand oleic acid makes the SnO(2) QDs less conducting. Ligand exchange of the long chain oleic acid (2.5 nm) by the short chain n-butylamine (0.6 nm) increases the current density of SnO(2) around 43 times due to the reduction of the interparticle distance. Ferromagnetic behaviour of oleic acid capped QDs may be ascribed to the defects in the host due to the alteration of the electronic structure owing to the capping.