No abstract

Citation for published item:rrenD FgF nd o¤ %t hovskyD uF nd hot nD rF nd veroyD gFwF nd gornuzD wF nd tell iD pF nd r¡ e ertD gF nd oths hildD eF nd qr¤ tzelD wF @PHIQA 9sdentifying h mpion n nostru tures for sol r w terEsplittingF9D x ture m teri lsFD IP @WAF ppF VRPEVRWF Further information on publisher's website: httpXGGdxFdoiForgGIHFIHQVGnm tQTVR Publisher's copyright statement: Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Charge transport in nanoparticle-based materials underlies many emerging energy conversion technologies, yet assessing the impact of nanometer-scale structure on charge transport across micron-scale distances remains a challenge. Here we develop an approach for correlating the spatial distribution of crystalline and current-carrying domains in entire nanoparticle aggregates. We apply this approach to nanoparticle-based α-Fe 2 O 3 electrodes that are of interest in solar-to-hydrogen energy conversion. In correlating structure and charge transport with nanometer resolution across micron-scale distances, we have identified the existence of champion nanoparticle aggregates that are most responsible for the high photoelectrochemical activity of the present electrodes. Indeed, when electrodes are fabricated with a high proportion of these champion nanostructures, the electrodes achieve the highest photocurrent of any metal oxide photoanode for photoelectrochemical water splitting under 100 mW cm -2 air mass 1.5 global sunlight.Batteries, fuel cells, and solar energy conversion devices have emerged as a class of important technologies that increasingly rely upon electrodes derived from nanoparticles 1 . These nanoparticle-based materials provide a unique challenge in assessing structure-property relationships because of the disordered arrangement of nanocrystals that results when nanoparticles collide and aggregate [2][3][4][5][6] . The morphological evolution that follows aggregation further obscures the influence of particle size, shape, and interfacial characteristics in defining the physical properties of these materials 7,8 . For the nanoparticle-based electrodes used in solar energy conversion, structural defects such as grain boundaries define pathways for charge transport by creating potential barriers and by promoting recombination 9 . Because of the complexity of these materials, within a single electrode there may exist a small proportion of "champion" nanostructures-by analogy with champion solar cells 10,11 , these are nanostructures that provide the highest solar conversion efficiencies-th...

Nanostructured semiconducting metal oxides and particularly single nanowire devices offer exceptional gas sensitivity but at the expense of statistical variations and excessive noise levels. In this study TiO2/poly(vinyl acetate) composite nanofiber mats were directly electrospun onto interdigitated Pt electrode arrays, hot pressed at 120 degrees C, and calcined at 450 degrees C. This resulted in a novel multiple nanowire network composed of sheaths of 200-500 nm diameter cores filled with readily gas accessible approximately 10 nm thick single-crystal anatase fibrils. TiO2 nanofiber sensors tested for NO2, in dry air, exhibited exceptional sensitivity showing with, for example, a 833% increase in sensor resistance when exposed to 500 ppb NO2 at 300 degrees C, consistent with a detection limit estimated to be well below 1 ppb. Unusual response patterns were observed at high NO2 concentrations (> 12.5 ppm), consistent with n to p inversion of the surface-trap limited conduction facilitated by the high surface-to-volume ratio of this material.

Semiconductor photoelectrodes for solar hydrogen production by water photoelectrolysis must employ stable, non-toxic, abundant and inexpensive visible-light absorbers. Iron oxide (α-Fe(2)O(3)) is one of few materials meeting these requirements, but its poor transport properties present challenges for efficient charge-carrier generation, separation, collection and injection. Here we show that these challenges can be addressed by means of resonant light trapping in ultrathin films designed as optical cavities. Interference between forward- and backward-propagating waves enhances the light absorption in quarter-wave or, in some cases, deeper subwavelength films, amplifying the intensity close to the surface wherein photogenerated minority charge carriers (holes) can reach the surface and oxidize water before recombination takes place. Combining this effect with photon retrapping schemes, such as using V-shaped cells, provides efficient light harvesting in ultrathin films of high internal quantum efficiency, overcoming the trade-off between light absorption and charge collection. A water photo-oxidation current density of 4 mA cm(-2) was achieved using a V-shaped cell comprising ~26-nm-thick Ti-doped α-Fe(2)O(3) films on back-reflector substrates coated with silver-gold alloy.

The effect of grain size on the sensitivity of chemoresistive nanocrystalline metal-oxide gas sensors was evaluated by calculating the effective carrier concentration as a function of the surface state density for a typical sensing material, SnO2, with different grain sizes between 5 and 80 nm. This involved numerical computation of the charge balance equation (the electroneutrality condition) using approximated analytical solutions of Poisson’s equation for small spherical crystallites. The calculations demonstrate a steep decrease in the carrier concentration when the surface state density reaches a critical value that corresponds to a condition of fully depleted grains, namely, when nearly all the electrons are trapped at the surface. Assuming that the variations in the surface state density are induced by surface interactions with ambient gas molecules, these calculations enable us to simulate the response curves of nanocrystalline gas sensors. The simulations show that the conductivity increases linearly with decreasing trapped charge densities, and that the sensitivity to the gas-induced variations in the trapped charge density is proportional to 1/D, where D is the average grain size.

Solar water splitting provides a promising path for sustainable hydrogen production and solar energy storage. One of the greatest challenges towards large-scale utilization of this technology is reducing the hydrogen production cost. The conventional electrolyser architecture, where hydrogen and oxygen are co-produced in the same cell, gives rise to critical challenges in photoelectrochemical water splitting cells that directly convert solar energy and water to hydrogen. Here we overcome these challenges by separating the hydrogen and oxygen cells. The ion exchange in our cells is mediated by auxiliary electrodes, and the cells are connected to each other only by metal wires, enabling centralized hydrogen production. We demonstrate hydrogen generation in separate cells with solar-to-hydrogen conversion efficiency of 7.5%, which can readily surpass 10% using standard commercial components. A basic cost comparison shows that our approach is competitive with conventional photoelectrochemical systems, enabling safe and potentially affordable solar hydrogen production.

The electronic structure, defect chemistry, and transport properties of members of the mixed ionic electronic conducting SrTi1 - x Fe x O3 - y (STF) solid-solution system are revisited, and an improved defect chemical model is proposed in which Fe is considered to be one of the main constituents that shape the energy-band structure of STF, rather than an impurity dopant with acceptor-like character. As a consequence of the high inherent deficiency in the oxygen sublattice, introduced by the mixed-valence states of the B-site cations Ti4+ and Fe3+, oxygen vacancies and interstitials generated by the anion Frenkel reaction dominate the defect equilibria, leading to predominant ionic conductivity at intermediate partial pressures of oxygen. Increasing Fe content results in both a systematic decrease in band-gap energy, E g 0 = 3.2 − 1.9x + 0.5x 2 eV, and reduction enthalpy, ΔH red = 5.8 − 3.4x + 1.7x 2 eV. The decrease in band gap is explained on the basis of the systematic broadening of the Fe-derived 3d band lying above the O-derived 2p valence band. The Fe-induced changes in band structure were experimentally confirmed by both optical and photoelectron (XPS) spectroscopies.

A 2-nm thick Nb(2)O(5) underlayer deposited by atomic layer deposition increases the charge separation efficiency and the photovoltage of ultrathin hematite films by suppressing electron back injection. Absorbed photon-to-current efficiencies (APCE) as high as 40%, which are one of the highest ever reported with hematite photoanodes, are obtained at 400 nm at +1.43 V vs. RHE.