Iridium oxide nanoparticles stabilized by a heteroleptic ruthenium tris(bipyridyl) dye were used as sensitizers in photoelectrochemical cells consisting of a nanocrystalline anatase anode and a Pt cathode. The dye coordinated the IrO(2) x nH(2)O nanoparticles through a malonate group and the porous TiO(2) electrode through phosphonate groups. Under visible illumination (lambda > 410 nm) in pH 5.75 aqueous buffer, oxygen was generated at anode potentials positive of -325 mV vs Ag/AgCl and hydrogen was generated at the cathode. The internal quantum yield for photocurrent generation was ca. 0.9%. Steady-state luminescence and time-resolved flash photolysis/transient absorbance experiments were done to measure the rates of forward and back electron transfer. The low quantum yield for overall water splitting in this system can be attributed to slow electron transfer (approximately 2.2 ms) from IrO(2) x nH(2)O to the oxidized dye. Forward electron transfer does not compete effectively with the back electron transfer reaction from TiO(2) to the oxidized dye, which occurred on a time scale of 0.37 ms.

Stable blue suspensions of 2 nm diameter iridium oxide (IrO x 3 nH 2 O) nanoparticles were obtained by hydrolyzing IrCl 6 2-in base at 90°C to produce [Ir(OH) 6 ] 2-and then treating with HNO 3 at 0°C. UV-visible spectra show that acid condensation of [Ir(OH) 6 ] 2-results in quantitative conversion to stable, ligand-free IrO x 3 nH 2 O nanoparticles, which have an extinction coefficient of 630 ( 50 M -1 cm -1 at 580 nm. In contrast, alkaline hydrolysis alone converts only 30% of the sample to IrO x 3 nH 2 O at 2 mM concentration. The acidified nanoparticles are stable for at least one month at 2°C and can be used to make colloidal solutions between pH 1 and 13. At pH 7 and above, some hydrolysis to form [Ir(OH) 6 ] 2-occurs. Uniform IrO x 3 nH 2 O electrode films were grown anodically from pH 1 solutions, and were found to be highly active for water oxidation between pH 1 and 13.SECTION: Nanoparticles and Nanostructures R ecent research activity in artificial photosynthesis has intensified the search for water oxidation catalysts that can function at high turnover rates and low overpotentials. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Despite the high cost and low terrestrial abundance of iridium, hydrated iridium oxide (IrO x 3 nH 2 O) has been useful for fundamental studies of the water splitting reaction because it can be made as stable nanoparticles, and because it is highly active for water oxidation over a broad range of pH. [16][17][18][19][20][21][22][23][24][25][26] The synthesis of IrO x 3 nH 2 O colloids was first reported over 100 years ago, 27 and that synthetic method (alkaline hydrolysis of [IrCl 6 ] 2-) produces blue colloids with particle sizes in the 1-2 nm range. Recently, Murray and co-workers used this method to deposit electrode films of IrO x 3 nH 2 O, which they showed are very good electrocatalysts over a broad range of pH. 22,28 Alternative syntheses of IrO x 3 nH 2 O colloids have used stabilizing ligands with multiple carboxylate groups, such as malonate or succinate. 29-32 With these stabilizing ligands, the colloids may be incorporated into photoelectrodes and other assemblies for overall light-driven water splitting. 23,33-35 In syntheses using capping ligands, the yield of stable colloid is rarely quantitative, and some of the IrO x 3 nH 2 O precipitates as large particles. Similarly, in our hands the alkaline route to uncapped colloids gives solutions of varying color, from pale to deep blue. Another complication of current synthetic methods is that IrO x 3 nH 2 O nanoparticles are not stable under acidic conditions; for example citrate-capped IrO x 3 nH 2 O precipitates at pH < 3, 36 and ligandfree IrO x 3 nH 2 O nanoparticles synthesized by alkaline hydrolysis are unstable at neutral pH. 22 Because of these problems, we have conducted a study of the alkaline hydrolysis process, the results of which are reported here. We identify conditions for obtaining quantitative yields of catalytically active, uncapped colloids that are stable over a wide range of pH.Hydrolysis o...

A facile, in-situ deposition route to stable iridium oxide (IrO(x)·nH(2)O) nanoparticle thin films from [Ir(OH)(6)](2-) solutions is reported. The [Ir(OH)(6)](2-) solution, made by alkaline hydrolysis of [IrCl(6)](2-), is colorless and stable near neutral pH, and forms blue IrO(x)·nH(2)O nanoparticle suspensions once it is adjusted to acidic or basic conditions. IrO(x)·nH(2)O nanoparticle thin films are grown anodically on glassy carbon, fluorine-doped tin oxide, and gold electrodes by electrolyzing [Ir(OH)(6)](2-) solutions at +1.0-1.3 V versus Ag/AgCl. The thickness of the IrO(x)·nH(2)O films can be controlled by varying the concentration of [Ir(OH)(6)](2-) , the deposition potential, and/or the deposition time. These thin films are stable between pH 1 and 13 and have the lowest overpotential (η) for the oxygen evolution reaction (OER) of any yet reported. Near neutral pH, the Tafel slope for the OER at a IrO(x)·nH(2)O film/Au rotating disk electrode was 37-39 mV per decade. The exchange current density for the OER was 4-8 × 10(-10) A cm(-2) at a 4 mC cm(-2) coverage of electroactive Ir.

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Photoelectrochemical water splitting occurs in a dye-sensitized solar cell when a [Ru(bpy)3]2+-based dye covalently links a porous TiO2 anode film to IrO2 x nH2O nanoparticles. The quantum yield for oxygen evolution is low because of rapid back electron transfer between TiO2 and the oxidized dye, which occurs on a timescale of hundreds of microseconds, When iodide is added as an electron donor, the photocurrent increases, confirming that the initial charge injection efficiency is high. When the porous TiO2 film is coated with a 1-2 nm thick layer of ZrO2 or Nb2O5, both the charge injection rate and back electron transfer rate decrease. The efficiency of the cell increases and then decreases with increasing film thickness, consistent with the trends in charge injection and recombination rates. The current efficiency for oxygen evolution, measured electrochemically in a generator-collector geometry, is close to 100%. The factors that lead to polarization of the photoanode and possible ways to re-design the system for higher efficiency are discussed.

The search for optimal thermoelectric materials aims for structures in which the crystalline order is disrupted to lower the thermal conductivity without degradation of the electron conductivity. Here we report the synthesis and characterisation of ternary nanoparticles (two cations and one anion) that exhibit a new form of crystalline order: an uninterrupted, perfect, global Bravais lattice, in which the two cations exhibit a wide array of distinct ordering patterns within the cation sublattice, forming interlaced domains and phases. Partitioning into domains and phases is not unique; the corresponding boundaries have no structural defects or strain and entail no energy cost. We call this form of crystalline order 'interlaced crystals' and present the example of hexagonal CuInS 2 . Interlacing is possible in multi-cation tetrahedrally bonded compound with an average of two electrons per bond. Interlacing has minimal effect on electronic properties, but should strongly reduce phonon transport, making interlaced crystals attractive for thermoelectric applications.

Herein, we report the unprecedented direct synthesis of a recently discovered metastable wurtzite phase of Cu2–x Se. Nanocrystals of Cu2–x Se were synthesized employing dodecyl diselenide as the selenium source and ligand. Optical characterization performed with UV–vis–NIR spectroscopy in solution showed a broad plasmonic band in the NIR. Structural characterization was performed with X-ray diffraction (XRD) and transmission electron microscopy. Variable-temperature XRD analysis revealed that the wurtzite nanocrystals irreversibly transform into the thermodynamic cubic phase at 151 °C. Replacement of dodecyl diselenide with dodecyl selenol yielded cubic phase Cu2–x Se, allowing for polymorphic phase control. An aliquot study was performed to gain insight into the mechanism of phase selectivity. The direct synthesis of this novel wurtzite phase could enable the discovery of new phenomena and expand the vast application space of Cu x Se y compounds.

Nanocrystals of CuInS 2 with the hexagonal wurtzite structure hold great potential for applications requiring efficient energy transport, such as photocatalysis, due to their anisotropic crystal structure. However, thus far their optical properties have proven difficult to study, as luminescence from wurtzite nanocrystals has only recently been observed. In this work, we report the colloidal synthesis of single crystalline, luminescent CuInS 2 nanocrystals with both the cubic and hexagonal structures. The crystalline phase, optical properties and mechanism of formation of nanocrystals are controlled by changing the reaction temperature. Photoluminescence is observed in the visible and near-infrared spectral regions, which results from the cubic and hexagonal nanocrystals respectively. Synthetic studies combined with XRD, TEM and EDS mapping provide evidence for the mechanisms behind phase selection. † Electronic supplementary information (ESI) available: Complete Rietveld refinement parameters; Scherrer line broadening crystallite size compared to TEM size; EDS data; quantum yields; Fityk peak fitting parameters; absorbance spectra of hexanes; TEM images from aliquot study; complete EDS maps for aliquot study, including dark field and elemental analysis data. See structure formed at these temperatures. 32 The QY was o0.8% for all samples (Fig. S5, ESI †).The visible peak, attributed to ZB CuInS 2 , was fit to a single Gaussian (Fig. S6, ESI †), which showed a red shift as reaction temperature increased. This is most likely due to an increase in the size of the NCs and the relaxation of quantum confinement conditions. This is consistent with the trends observed in absorbance spectra and reported in literature examples. [56][57][58] The broad, multimodal shape of the NIR PL peak, centred at B950 nm, and the large Stokes shift (0.25 eV) indicate that Fig. 3 (a) XRD of CuInS 2 NCs prepared at various temperatures. Pure ZB and WZ spectra are digitized from the work of Chang et al.; 36 (b) Proportion of WZ (blue), ZB (red) and hiCC Cu 2 S (green) phases present in each sample plotted as a function of temperature, as determined by Rietveld refinement of XRD; (c) EDS map of CuInS 2 NCs prepared at 155 1C showing the presence of Cu 2 S NCs; (d), (e) High resolution TEM (HRTEM) images of CuInS 2 NCs prepared at 215 1C (plates) and 115 1C (spheres) respectively showing lattice fringing.Fig. 4 (a) Absorbance spectra of CuInS 2 NC dispersions in hexanes prepared at different temperatures; (b) Tauc plot of the predominantly WZ CuInS 2 NCs prepared at 215 1C; (c) PL spectra normalized to the QY of CuInS 2 NC dispersions in hexanes prepared at different temperatures.