Co-reporter:Verónica Celorrio, Paola M. Quaino, Elizabeth Santos, Jonathan Flórez-Montaño, Jo J. L. Humphrey, Olmedo Guillén-Villafuerte, Daniela Plana, María J. Lázaro, Elena Pastor, and David J. Fermín
ACS Catalysis March 3, 2017 Volume 7(Issue 3) pp:1673-1673
Publication Date(Web):January 17, 2017
DOI:10.1021/acscatal.6b03237
The mechanism of CO and HCOOH electrooxidation in an acidic solution on carbon-supported Au–Pd core–shell nanoparticles was investigated by differential electrochemical mass spectrometry and in situ Fourier transform infrared (FTIR) spectroscopy. Analysis performed in nanostructures with 1.3 ± 0.1 nm (CS1) and 9.9 ± 1.1 nm (CS10) Pd shells provides compelling evidence that the mechanism of adsorbed CO (COads) oxidation is affected by structural and electronic effects introduced by the Au cores. In the case of CS10, a band associated with adsorbed OH species (OHads) is observed in the potential range of CO oxidation. This feature is not detected in the case of CS1, suggesting that the reaction follows an alternative mechanism involving COOHads species. The faradaic charge associated with COads oxidation as well as the Stark slope measured from FTIR indicates that the overall affinity and orbital coupling of CO to Pd are weaker for CS1 shells. FTIR spectroscopy also revealed the presence of HCOOads intermediate species only in the case of CS1. This observation allowed us to conclude that the higher activity of CS10 toward this reaction is due to a fast HCOOads oxidation step, probably involving OHads, to generate CO2. Density functional theory calculations are used to estimate the contributions of the so-called ligand and strain effects on the local density of states of the Pd d-band. The calculations strongly suggest that the key parameter contributing to the change in mechanism is the effective lattice strain.Keywords: Au−Pd core−shells; DEMS; formic acid; in situ FTIR; strain effect;
Co-reporter:Devendra Tiwari, Tristan Koehler, Xianzhong Lin, Andrei SaruaRobert Harniman, Lan Wang, Reiner Klenk, David J Fermin
ACS Applied Materials & Interfaces 2017 Volume 9(Issue 3) pp:
Publication Date(Web):December 29, 2016
DOI:10.1021/acsami.6b12306
A single molecular precursor solution is described for the deposition of CuIn(S,Se)2 (CIS) film onto Mo-coated glass substrates by spin coating, followed by annealing in Se atmosphere. Characterization of the films by X-ray diffraction, Raman spectroscopy and scanning electron microscopy demonstrates the formation of a highly homogeneous and compact 1.1 μm thick CIS layer, with a MoSe2 under-layer. Atomic force microscopy reveals the presence of spherical grains between 400 and 450 nm, featuring surface corrugation in the range of 30 nm. Film composition is found to be in close agreement with that of the precursor solution. Diffuse reflectance spectroscopy shows a direct band gap (Eg) of 1.36 eV. Intensity and temperature dependence photoluminescence spectra show characteristic features associated with a donor–acceptor pair recombination mechanism, featuring activation energy of 34 meV. Over 85 solar cell devices with the configuration Mo/CIS/CdS/i-ZnO/Al:ZnO/Ni–Al and an total area of 0.5 cm2 were fabricated and tested. The champion cell shows a power efficiency of 3.4% with an open circuit voltage of 521 mV and short circuit current of 14 mA/cm2 under AM 1.5 illumination and an external quantum efficiency above 60%. Overall variation in each of solar cell parameters remains below 10% of the average value, demonstrating the remarkable homogeneity of this solution processing method. To understand the limitation of devices, the dependence of the open-circuit voltage and impedance spectra upon temperature were analyzed. The data reveal that the CuIn(S,Se)2/CdS interface is the main recombination pathway with an activation energy of 0.79 eV as well as the presence of two “bulk” defect states with activation energies of 37 and 122 meV. We also estimated that the MoSe2 under-layer generates back contact barrier of 195 meV.Keywords: CuIn(S,Se)2; impedance spectroscopy; recombination losses; solar cells; solution processing; thin films;
Co-reporter:Devendra Tiwari;Ekaterina Skidchenko;Jake W. Bowers;Michael V. Yakushev;Robert W. Martin
Journal of Materials Chemistry C 2017 vol. 5(Issue 48) pp:12720-12727
Publication Date(Web):2017/12/14
DOI:10.1039/C7TC03953K
The nature and dynamics of acceptor states in solution-processed Cu2ZnSn(S,Se)4 (CZTSSe) thin films are investigated by variable temperature photoluminescence (PL) and electrical impedance spectroscopy. Highly pure I-4 phase CZTSSe with the composition Cu1.6ZnSn0.9(S0.23Se0.77)4 is synthesized by sequentially spin coating of dimethyl-formamide/isopropanol solutions containing metal salts and thiourea onto Mo coated glass, followed by annealing in an Se atmosphere at 540 °C. As-annealed films are highly compact with a thickness of 1.3 μm and grain sizes above 800 nm, with a band gap of 1.18 eV. Photovoltaic devices of 0.25 cm2 with the architecture glass/Mo/CZTSSe/CdS/i-ZnO/Al:ZnO demonstrate a power conversion efficiency reaching up to 5.7% in the absence of an anti-reflective coating. Under AM 1.5G illumination at 296 K, the best device shows a 396 mV open-circuit voltage (VOC), 27.8 mA cm−2 short-circuit current (JSC) and 52% fill factor (FF). The overall dispersion of these parameters is under 15% for a total of 20 devices. In the near IR region, PL spectra are dominated by two broad and asymmetrical bands at 1.14 eV (PL1) and 0.95 eV (PL2) with characteristic power and temperature dependences. Analysis of the device electrical impedance spectra also reveals two electron acceptor states with the same activation energy as those observed by PL. This allows assigning PL1 as a radiative recombination at localized copper vacancies (VCu), while PL2 is associated with CuZn antisites, broadened by potential fluctuations (band tails). The impact of these states on device performance as well as other parameters, such as barrier collection heights introduced by partial selenization of the back contact, are discussed.
Co-reporter:Devendra Tiwari;Tristan Koehler;Reiner Klenk
Sustainable Energy & Fuels (2017-Present) 2017 vol. 1(Issue 4) pp:899-906
Publication Date(Web):2017/05/30
DOI:10.1039/C7SE00150A
High quality microcrystalline tetragonal p-Cu2SnS3 (CTS) films are prepared by spin-coating a single precursor of metal salts and thiourea on to 5 × 5 cm2 Mo substrates. The best of the solar cells completed on these films with a substrate configuration: glass/Mo/CTS/CdS/i-ZnO/Al:ZnO/Ni–Al and a total area of 0.5 cm2 displays an open-circuit voltage of 206 mV, a fill factor of 34.5%, a short circuit current density of 27.6 mA cm−2 and a power conversion efficiency of 1.9% under simulated AM1.5 illumination. This the best performance reported for such solar architectures obtained by solution processing, with dispersion below 20% for 24 devices. For the first time, the key factors limiting CTS device efficiency are quantitatively established based on temperature dependent current–voltage curves and impedance spectroscopy, namely: (i) carrier recombination at the CTS/CdS interface, (ii) MoS2 non-ohmic back contact, formed due to sulfurization of the top Mo layer, with a barrier height of up to 216 ± 14 meV and (iii) the presence of two trap levels with activation energies 41 ± 0.4 meV and 206 ± 7 meV. The shallower trap is linked Cu vacancies, while the deeper trap is associated with Sn in Cu antisite defects based on DFT supercell calculations.
Co-reporter:María Isabel Díez-García, Verónica Celorrio, Laura Calvillo, Devendra Tiwari, Roberto Gómez, David J. Fermín
Electrochimica Acta 2017 Volume 246(Volume 246) pp:
Publication Date(Web):20 August 2017
DOI:10.1016/j.electacta.2017.06.025
The behavior of YFeO3 thin-film electrodes under illumination is investigated for the first time. YFeO3 thin films on F-doped SnO2 (FTO) electrodes were prepared by two different methods (A) deposition of nanoparticles synthesized by the so-called ionic liquid route at 1000° C followed by sintering at 400° C and (B) spin coating of a sol-gel precursor followed by a heat treatment at 640° C. Method A provides highly texture films with exquisite orthorhombic phase purity and a direct band gap transition at 2.45 eV. On the other hand, method B results in very compact and amorphous films. XPS confirmed a Fe3+ oxidation state in both films, with a surface composition ratio of 70:30 Y:Fe. Both materials exhibit cathodic photocurrent responses arising from hydrogen evolution in alkaline solutions with an onset potential of 1.05 V vs. RHE. The complex behavior of the photoresponses is rationalized in terms of recombination losses, band edge energy tails and hindered transport across the oxide thin film.
Co-reporter:Devendra Tiwari, David J Fermin
Electrochimica Acta 2017 Volume 254(Volume 254) pp:
Publication Date(Web):10 November 2017
DOI:10.1016/j.electacta.2017.09.110
High quality PbI2 films are prepared by a new route based on iodination of solution processed PbS films at room temperature. The PbI2 films are characterized by a highly textured morphology with flake-like particles sizing 500 and 800 nm across. Quantitative X-ray diffraction and resonance Raman spectroscopy demonstrate the crystallization of highly phase pure 2H-PbI2 with lattice parameters: a = 4.5505(9) Å, c = 6.9784(14) Å. Electronic structures and vibrational modes are calculated employing DFT. Transmittance and reflectance measurements are characterized by a sharp absorption edge corresponding to a direct band gap transition of 2.41 eV. Photoelectrochemical measurements carried out in 0.1 M KI solution showed that the material behaves as a p-type semiconductor with an apparent donor concentration of the order of 1015 cm−3 as well as stable photoresponses associated with hydrogen evolution with nearly 50% incident photon-to-current efficiency. Quantitative analysis of the spectral and potential dependence of the photocurrent responses shows that surface recombination is negligible even at the flat band potential. We conclude that I− species stabilizes the semiconductor surface, generating an ideal semiconductor/electrolyte junction, while enabling the transfer of minority carrier from the conduction band edge dominated by lead antibonding p-orbitals
Co-reporter:Alicia Gomis-Berenguer, Verónica Celorrio, Jesus Iniesta, David J. Fermin, Conchi O. Ania
Carbon 2016 Volume 108() pp:471-479
Publication Date(Web):November 2016
DOI:10.1016/j.carbon.2016.07.045
This work provides new insights in the field of applied photoelectro chemistry based on the use of nanoporous carbons as additives to tungsten oxide for the photooxidation of water under potential bias. Using a nanoporous carbon of low surface functionalization as additive to WO3 we have shown the dependence of the photochemical oxidation of water with the wavelength of the irradiation source. Photoelectrochemical responses obtained under monochromatic illumination show a significant increase in the incident photon-to-current conversion efficiency (IPCE) values for electrodes featuring up to 20 wt% carbon additive. Photoelectrochemical transient responses also show a sharp potential dependence, suggesting that the performance of the electrodes is strongly influenced by the carrier mobility and recombination losses. Despite the modest IPCE values of the W/NC electrodes (due to high bulk recombination and poor electron transport properties of the electrodes), our data shows that the incorporation of an optimal amount of nanoporous carbon additive to WO3 can enhance the carrier mobility of the semiconductor, without promoting additional recombination pathways or shadowing of the photoactive oxide.
Co-reporter:D. Plana, K. A. Bradley, D. Tiwari and D. J. Fermín
Physical Chemistry Chemical Physics 2016 vol. 18(Issue 18) pp:12428-12433
Publication Date(Web):15 Apr 2016
DOI:10.1039/C6CP02231F
The efficiency of photoelectrochemical reactions is conventionally defined in terms of the ratio between the current responses arising from the collection of carriers at electrical contacts and the incident photon flux at a given wavelength, i.e. the incident-photon-to-current-efficiency (IPCE). IPCE values are determined by a variety of factors such as the absorption constant of the active layer, bulk and surface recombination of photogenerated carriers, as well as their characteristic diffusion length. These parameters are particularly crucial in nanostructured photoelectrodes, which commonly display low carrier mobility. In this article, we examine the photoelectrochemical responses of a mesoporous TiO2 film in which the IPCE is enhanced by fast extraction of carriers via chemical reactions. TiO2 films are spontaneously formed by destabilisation of colloidal particles at the polarisable interface between two immiscible electrolyte solutions. The photocurrent arises from hole-transfer to redox species confined to the organic electrolyte, which is coupled to the transfer of electrons to oxygen in the aqueous electrolyte. The dynamic photocurrent responses demonstrate that no coupled ion transfer is involved in the process. The interplay of different interfacial length scales, molecularly sharp liquid/liquid boundary and mesoporous TiO2 film, promotes efficiencies above 75% (without correction for reflection losses). This is a significant step change in values reported for these interfaces (below 1%), which are usually limited to sub-monolayer coverage of photoactive molecular or nanoscopic materials.
Co-reporter:Dr. Verónica Celorrio;Ellie Dann;Dr. Laura Calvillo;Dr. David J. Morgan;Dr. Simon R. Hall;Dr. David J. Fermin
ChemElectroChem 2016 Volume 3( Issue 2) pp:283-291
Publication Date(Web):
DOI:10.1002/celc.201500440
Abstract
The kinetics of the oxygen reduction reaction (ORR) at carbon-supported transition-metal oxides in alkaline solutions is systematically investigated as a function of the nature of the B-site. The study is focused on LaBO3 (B=Cr, Co, Fe, Mn and Ni) nanoparticles synthesized by using an ionic-liquid route, offering fine control over phase purity and composition. Activity towards the ORR was compared with the commercial Pt/Etek catalyst. Detailed electrochemical analysis employing a rotating ring-disk electrode provides conclusive evidence that the carbon support plays an important contribution in the faradaic responses. Decoupling the contribution of the carbon support uncovers that the reactivity of LaMnO3 towards the four-electron ORR pathway is orders of magnitude higher than that for the other lanthanides. We rationalize these observations in terms of changes in the redox state at the B-site close to the formal oxygen reduction potential.
Co-reporter:Jo J. L. Humphrey;Dr. Daniela Plana;Dr. Verónica Celorrio;Dr. Sajanikumari Sadasivan;Dr. Robert P. Tooze;Dr. Paramaconi Rodríguez; David J. Fermín
ChemCatChem 2016 Volume 8( Issue 5) pp:952-960
Publication Date(Web):
DOI:10.1002/cctc.201501260
Abstract
The electrocatalytic reduction of CO2 at carbon-supported Au-Pd core–shell nanoparticles is investigated systematically as a function of the Pd shell thickness. Liquid- and gas-phase products were determined by off-line 1H NMR spectroscopy and on-line electrochemical mass spectrometry. Our results uncover the relationship between the nature of the products generated and the Pd shell thickness. CO and H2 are the only products generated at 1 nm thick shells, whereas shells of 5 and 10 nm produced HCOO−, CH4 and C2H6. The concentration of HCOO− detected in the electrolyte was dependent on the applied potential and reached a maximum Faradaic efficiency of 27 % at −0.5 V versus the reversible hydrogen electrode for 10 nm thick shells. We conclude that collisions between absorbed hydrogen at relaxed Pd lattices and strongly bound “CO-like” intermediates promote the complete hydrogenation to C1 and C2 alkanes without the generation of other products, such as alcohols and aldehydes.
Co-reporter:Veronica Celorrio, Devendra Tiwari, and David J. Fermin
The Journal of Physical Chemistry C 2016 Volume 120(Issue 39) pp:22291-22297
Publication Date(Web):September 13, 2016
DOI:10.1021/acs.jpcc.6b04781
The electrochemical activity toward the oxygen reduction reaction (ORR) of carbon supported Ba0.5Sr0.5CoxFe1–xO3−δ electrode particles is studied for the first time as a function of chemical composition (1 < x < 0). Highly crystalline Ba0.5Sr0.5CoxFe1–xO3−δ particles with the full compositional range were synthesized by an ionic-liquid route, leading to a high degree of phase purity. The structure and composition of the materials were investigated by quantitative XRD and EDX. Electrochemical studies in oxygen-saturated alkaline solutions show that the overall reactivity of the catalyst supported on mesoporous carbon increases as the Co content increases above 50%. DFT calculations, based on the structural parameters obtained from XRD refinements, show that the oxygen-vacancy formation energy decreases as the Co content increases. Oxygen vacancies lead to a higher electron density at the Co atoms, which are identified as the key ORR active sites.
Co-reporter:Thomas A. G. Risbridger, Daniel W. Watkins, James P. K. Armstrong, Adam W. Perriman, J. L. Ross Anderson, and David J. Fermin
Biomacromolecules 2016 Volume 17(Issue 11) pp:3485
Publication Date(Web):September 20, 2016
DOI:10.1021/acs.biomac.6b00928
The modification of protein surfaces employing cationic and anionic species enables the assembly of these biomaterials into highly sophisticated hierarchical structures. Such modifications can allow bioconjugates to retain or amplify their functionalities under conditions in which their native structure would be severely compromised. In this work, we assess the effect of this type of bioconjugation on the redox properties of two model heme proteins, that is, cytochrome c (CytC) and myoglobin (Mb). In particular, the work focuses on the sequential modification by 3-dimethylamino propylamine (DMAPA) and 4-nonylphenyl 3-sulfopropyl ether (S1) anionic surfactant. Bioconjugation with DMAPA and S1 are the initial steps in the generation of pure liquid proteins, which remain active in the absence of water and up to temperatures above 150 °C. Thin-layer spectroelectrochemistry reveals that DMAPA cationization leads to a distribution of bioconjugate structures featuring reduction potentials shifted up to 380 mV more negative than the native proteins. Analysis based on circular dichroism, MALDI-TOF mass spectrometry, and zeta potential measurements suggest that the shift in the reduction potentials are not linked to protein denaturation, but to changes in the spin state of the heme. These alterations of the spin states originate from subtle structural changes induced by DMAPA attachment. Interestingly, electrostatic coupling of anionic surfactant S1 shifts the reduction potential closer to that of the native protein, demonstrating that the modifications of the heme electronic configuration are linked to surface charges.
Co-reporter:H. Zanin, P.W. May, R.L. Harniman, T. Risbridger, E.J. Corat, D.J. Fermin
Carbon 2015 Volume 82() pp:288-296
Publication Date(Web):February 2015
DOI:10.1016/j.carbon.2014.10.073
Electrochemically active diamond-like carbon (DLC) electrodes featuring high specific surface area have been prepared by plasma-enhanced chemical vapour deposition (CVD) onto densely packed forests of vertically aligned multiwall carbon nanotubes (VACNTs). The DLC:VACNT composite film exhibits a complex topography with web like features and ridges generated by partial coalescence of the DLC over the CNT arrays. DLC:VACNT electrodes exhibit low background responses over a large potential window, low uncompensated resistance, as well as low charge-transfer impedance in the presence of ferrocyanide as a redox probe. The interfacial capacitance associated with the DLC:VACNT electrode is in the range of 0.6 mF cm−2, a value two orders of magnitude larger than in conventional flat carbon electrodes. DLC films grown onto single-crystal Si(1 0 0) under identical conditions resulted in essentially insulating layers. Conducting-atomic force microscopy studies reveal that the film electro-activity does not arise from specific topographic features in the highly corrugated film. The ensemble of experimental results suggests that the enhanced electrochemical responses are not connected to areas in which the CNT support is exposed to the electrolyte solution. This is remarkable behaviour considering that no dopants have been included during the DLC film growth.
Co-reporter:Jo J. L. Humphrey;Dr. Sajanikumari Sadasivan;Dr. Daniela Plana;Dr. Verónica Celorrio;Dr. Robert A. Tooze; David J. Fermín
Chemistry - A European Journal 2015 Volume 21( Issue 36) pp:12694-12701
Publication Date(Web):
DOI:10.1002/chem.201501496
Abstract
Oleylamine (OA) based “hot injection” colloidal synthesis offers a versatile approach to the synthesis of highly monodisperse metallic and multi-metallic alloyed nanostructures in the absence of potentially toxic and unstable phosphine compounds. For application in heterogeneous catalysis and electrocatalysis, the adsorbed OA species at the metal surfaces should be effectively removed without compromising the structure and composition of the nanostructures. Herein, we investigate the removal of OA from colloidal Pt nanoparticles through 1) “chemical methods” such as washing in acetic acid or ethanol, and ligand exchange with pyridine; and 2) thermal pre-treatment between 185 and 400 °C in air, H2 or Ar atmospheres. The electrochemical reactivity of Pt nanoparticles is acutely affected by the presence of surface organic impurities, making this material ideal for monitoring the effectiveness of OA removal. The results showed that thermal treatment in Ar at temperatures above 400 °C provides highly active particles, with reactivity comparable to the benchmark commercial catalyst, Pt/ETEK. The mechanism involved in thermal desorption of OA was also investigated by thermogravimetric analysis coupled to mass spectrometry (TGA-MS). Oxidation of HCOOH and adsorbed CO in acidic solution were used as test reactions to assess the Pt electrocatalytic activity.
Co-reporter:Bo Hou, David Benito-Alifonso, Richard Webster, David Cherns, M. Carmen Galan and David J. Fermín
Journal of Materials Chemistry A 2014 vol. 2(Issue 19) pp:6879-6886
Publication Date(Web):19 Feb 2014
DOI:10.1039/C4TA00285G
The replacement of phosphine containing compounds in the synthesis of II–VI quantum dots (QDs) via the ‘hot-injection’ method has received considerable attention in recent years, in particular toward scaling-up production. A key bottleneck in current approaches is the poor solubility of elemental Se in solvents such as 1-octadecene (1-ODE) or oleylamine requiring a heating temperature of 200 °C for several hours, or the introduction of additives such as alkylthiols or NaBH4, or replacement by SeO2. In the present work, we elucidate the fundamental steps in the dissolution of elemental Se in 1-ODE and oleylamine with the view of facilitating the large-scale synthesis of CdSe QDs. The main organoselenium species generated during the solubilisation of elemental Se in 1-ODE and oleylamine were identified by 1D and 2D NMR spectroscopy (1H, 13C and 77Se). Experimental evidence suggests that the rate determining step is the formation of Se radicals, via homolytic cleavage of the Se–Se bond, that attack the allylic proton in 1-ODE and oleylamine. Plausible reaction pathways in both systems are proposed. Finally, we demonstrate that the radical-mediated solubilisation of Se can be significantly accelerated by the addition of azobisisobutyronitrile (AIBN), a common radical initiator used in the polymer industry. In this way, a highly concentrated Se precursor was prepared. The “hot-injection” of the Se precursor into CdO containing 1-ODE solution leads to the formation of highly luminescent CdSe QDs with a well-defined cubic structure.
Co-reporter:Dr. Verónica Celorrio;Kieren Bradley;Oliver J. Weber;Dr. Simon R. Hall;Dr. David J. Fermín
ChemElectroChem 2014 Volume 1( Issue 10) pp:1667-1671
Publication Date(Web):
DOI:10.1002/celc.201402192
Abstract
The photoelectrochemical properties of LaFeO3 nanoparticles are discussed for the first time. Highly phase-pure LaFeO3 particles prepared by a novel ionic-liquid-based method are characterized by a (60±14) nm mean size and a band gap of (2.56±0.07) eV. Thin films deposited by screen printing exhibit photocurrent responses associated with hydrogen generation at potential up to 1 V more positive than the formal hydrogen potential. Analysis of the photocurrent responses as a function of photon flux and potential bias suggest that the hole-collection efficiency at the back contact is in competition with interfacial water oxidation.
Co-reporter:Qian Zhang ; Verónica Celorrio ; Kieren Bradley ; Flurin Eisner ; David Cherns ; Wei Yan ;David J. Fermín
The Journal of Physical Chemistry C 2014 Volume 118(Issue 31) pp:18207-18213
Publication Date(Web):July 9, 2014
DOI:10.1021/jp505091t
Correlations between the population of deep trap states in an array of TiO2 nanotubes (NT) and the dynamic photocurrent responses under supra-band-gap illumination are investigated. Ordered arrays of TiO2 NT of 10 μm length, 125 nm inner diameter, and 12 nm wall thickness featuring strong anatase character were obtained by anodization of Ti in ethylene glycol solution containing NH4F. Cyclic voltammograms at pH 10 show the characteristic responses for nanostructured TiO2 electrodes, in particular a sharp cathodic peak as the electron density in the film increases. These responses are associated with the population of deep trap states with an average value of 5 × 104 electrons per NT. Dynamic photocurrent measurements clearly show that the characteristic rise time of the photocurrent increases as the potential is increased above the onset region for charging deep trap states. At potentials in which deep trap states are fully depopulated in the dark, photocurrent rise time approaches values just below 1 s, which is more than 3 orders of magnitude slower than the estimated RC time constant. The occupancy of the deep trap states under photostationary conditions is a fraction of the density of states estimated from voltammetric responses. These findings are discussed in the context of current views about trap states in high surface area TiO2 electrodes.
Co-reporter:Kamendra P. Sharma, Kieren Bradley, Alex P. S. Brogan, Stephen Mann, Adam W. Perriman, and David J. Fermin
Journal of the American Chemical Society 2013 Volume 135(Issue 49) pp:18311-18314
Publication Date(Web):November 18, 2013
DOI:10.1021/ja4104606
Redox responses associated with the heme prosthetic group in a myoglobin-polymer surfactant solvent-free liquid are investigated for the first time in the absence of an electrolyte solution. Cyclic voltammograms from the biofluid exhibit responses that are consistent with planar diffusion of mobile charges in the melt. Temperature-dependent dynamic electrochemical and rheological responses are rationalized in terms of the effective electron hopping rate between heme centers and the transport of intrinsic ionic species in the viscous protein liquid.
Co-reporter:D. Plana, J. J. L. Humphrey, K. A. Bradley, V. Celorrio, and D. J. Fermín
ACS Applied Materials & Interfaces 2013 Volume 5(Issue 8) pp:2985
Publication Date(Web):March 19, 2013
DOI:10.1021/am302397p
High surface area composites featuring metal nanostructures and diamond particles have generated a lot of interest in the fields of heterogeneous catalysis, electrocatalysis, and sensors. Diamond surfaces provide a chemically robust framework for active nanostructures in comparison with sp2 carbon supports. The present paper investigates the charge transport properties of high surface area films of high-pressure, high-temperature diamond particles in the presence and absence of metal nanostructures, employing electrochemical field-effect transistors. Oxygen- and hydrogen-terminated surfaces were generated on 500 nm diamond powders. Homogeneously distributed metal nanostructures, with metal volume fractions between ca. 5 and 20%, were either nucleated at the diamond particles by impregnation or incorporated from colloidal solution. Electrochemical field-effect transistor measurements, employing interdigitated electrodes, allowed the determination of composite conductivity as a function of electrode potential, as well as in air. In the absence of metal nanostructures, the lateral conductivity of the diamond assemblies in air is increased by over one order of magnitude upon hydrogenation of the particle surface. This observation is consistent with studies at diamond single crystals, although the somewhat modest change in conductivity suggests that charge transport is not only determined by the intrinsic surface conductivity of individual diamond particles but also by particle-to-particle charge transfer. Interestingly, the latter contribution effectively controls the assembly conductivity in the presence of an electrolyte solution as the difference between hydrogenated and oxygenated particles vanishes. The conductivity in the presence of metal nanoparticles is mainly determined by the metal volume fraction, while diamond surface termination and the presence of electrolyte solutions exert only minor effects. The experimental trends are discussed in terms of the electrochemical formation of charge carriers in the diamond particles, percolation theory, and charge screening at the double layer.Keywords: charge transport; high-pressure high-temperature nanodiamond; nanocomposites; nanoparticles; percolation theory;
Co-reporter:Daniela Plana, Jonathan Flórez-Montaño, Veronica Celorrio, Elena Pastor and David J. Fermín
Chemical Communications 2013 vol. 49(Issue 93) pp:10962-10964
Publication Date(Web):14 Oct 2013
DOI:10.1039/C3CC46543H
The faradaic efficiency of CO2 electroreduction is significantly affected by the thickness of Pd nanoshells on Au cores. The ratio of hydrogen evolution to CO2 reduction was determined by differential electrochemical mass spectrometry. Decreasing the Pd shell thickness from 10 to 1 nm leads to a twofold increase in faradaic efficiency.
Co-reporter:Jun Wang, Joe Harris, Mei Li, Daniela Plana, Michele E. Barbour, David J. Fermin and Stephen Mann
CrystEngComm 2013 vol. 15(Issue 36) pp:7152-7156
Publication Date(Web):30 May 2013
DOI:10.1039/C3CE40704G
Macroporous biomineralized composites in the form of thin slices of tooth dentine were used to prepare oriented arrays of high aspect ratio copper microwires by template-directed electrochemical deposition. The coaligned wires were 1 to 4 μm in thickness, and spatially separated throughout the dentine matrix to produce a metallized inorganic–organic biocomposite that exhibited ohmic conductivity and enhanced mechanical hardness. Utilization of porous biomineral templates for the crystallization of metallic microwire arrays offers a novel step towards the low temperature fabrication of multi-functional conductive hybrid composites with integrated bioinspired properties.
Co-reporter:V. Celorrio ; D. Plana ; J. Flórez-Montaño ; M. G. Montes de Oca ; A. Moore ; M. J. Lázaro ; E. Pastor ;D. J. Fermín
The Journal of Physical Chemistry C 2013 Volume 117(Issue 42) pp:21735-21742
Publication Date(Web):September 24, 2013
DOI:10.1021/jp4039804
The electrocatalytic reactivity of Pt nanoparticles supported on high-pressure, high-temperature diamond particles toward adsorbed CO, methanol, and formic acid oxidation is investigated employing differential electrochemical mass spectrometry (DEMS). Surface treatment of diamond particles, employed as dimensionally stable electrocatalyst supports, leads to materials with surfaces featuring mainly hydrogen (HDP) or oxygen-based functional groups (ODP). Pt nanoparticles with average diameter below 5 nm were generated by impregnation of the modified diamond particles. The voltammetric responses associated with the oxidation of adsorbed CO appeared unaffected by the surface termination of the diamond support. However, significant differences were observed for methanol oxidation in acid solutions, with Pt/HDP producing smaller current densities than Pt/ODP and a commercially available Pt catalyst (Pt/E-TEK). DEMS studies show higher conversion efficiencies to CO2 for Pt/ODP and Pt/E-TEK, while Pt/HDP exhibited values of approximately 90%. Evidence of formic acid generation as intermediate during methanol oxidation was obtained on all catalysts. Significant differences in the current density associated with the oxidation of formic acid were also observed, with Pt/HDP also providing the lowest current densities. The ensemble of the experimental data suggests that adsorbed HCOOads species is the key intermediate in methanol oxidation, and its subsequent oxidation to CO2 is strongly affected by the effective surface termination of the diamond support.
Co-reporter:Bo Hou ; David Parker ; Gabriela P. Kissling ; Jonathan A. Jones ; David Cherns ;David J. Fermín
The Journal of Physical Chemistry C 2013 Volume 117(Issue 13) pp:6814-6820
Publication Date(Web):March 7, 2013
DOI:10.1021/jp400208a
CdSe1–xTex quantum dot (QD) alloys are characterized by high luminescence quantum yields and a strong band gap bowing as a function of the Se:Te ratio, featuring longer emission wavelengths than CdTe or CdSe dots of identical size. In this contribution, these properties are rationalized by examining the structure and band edge energy of CdSe1–xTex as functions of x. The QDs were synthesized employing the “hot-injection” method, in the presence of either trioctylphosphine oxide (TOPO) or octadecene (ODE) as the Cd precursor solvent. Elementary analysis of the QDs indicated that TOPO plays a crucial role in tuning the content of Se in the alloys, as only traces of this element were found when using ODE. Detailed studies based on X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) revealed a high degree of complexity in the structure of the alloyed dots. The analysis concluded that the structure of the QDs was essentially wurtzite, although features associated with zinc blende can be seen due to the presence of stacking faults and to a small population of nanocrystals with cubic structure. More importantly, these studies reveal a nonlinear expansion of the effective lattice constant with increasing Te content. The valence band edge energy of the alloys in solution was estimated from the first oxidation potential measured by linear sweep voltammetry at Au microelectrodes. The results show that the valence band edge exhibits a very weak dependence on x for values below 0.5, indicating that the decrease in the optical band gap is mainly linked to a decrease in the conduction band edge energy. For x > 0.5, the conduction and valence band edges shift to higher values with an overall increase in the band gap. The experimental trends show, for the first time, that the characteristic red shift of the band gap with low to intermediate Te content is determined by relaxation of the lattice constant, whereas the contribution arising from the change in anion electronegativity becomes predominant for x > 0.5.
Co-reporter:Bo Hou;Dr. David Benito-Alifonso;Nessrin Kattan; David Cherns;Dr. M. Carmen Galan; David J. Fermín
Chemistry - A European Journal 2013 Volume 19( Issue 47) pp:15847-15851
Publication Date(Web):
DOI:10.1002/chem.201302722
Co-reporter:V. Celorrio, M.G. Montes de Oca, D. Plana, R. Moliner, D.J. Fermín, M.J. Lázaro
International Journal of Hydrogen Energy 2012 Volume 37(Issue 8) pp:7152-7160
Publication Date(Web):April 2012
DOI:10.1016/j.ijhydene.2011.12.014
Co-reporter:Dr. Wiphada Hongthani;Dr. Avinash J. Patil; Stephen Mann ;Dr. David J. Fermín
ChemPhysChem 2012 Volume 13( Issue 12) pp:
Publication Date(Web):
DOI:10.1002/cphc.201290056
Co-reporter:Dr. Wiphada Hongthani;Dr. Avinash J. Patil; Stephen Mann ;Dr. David J. Fermín
ChemPhysChem 2012 Volume 13( Issue 12) pp:2956-2963
Publication Date(Web):
DOI:10.1002/cphc.201200407
Abstract
The properties of graphene oxide (GO) and DNA-stabilised reduced graphene-oxide (rGO) sheets as electron-transfer mediators in partially blocked electrodes are evaluated employing electrochemical impedance spectroscopy. Evidences obtained from UV/Vis, Raman and FTIR spectroscopies, as well as atomic force microscopy, confirm that the reduction of exfoliated GO single sheets by hydrazine yields partially reduced graphene oxide featuring a high defect density. Two-dimensional assemblies of GO and rGO were formed through electrostatic adsorption at Au electrodes, sequentially modified with 11-mercaptoundecanoic acid (MUA) and poly-diallyldimethylammonium chloride (PDADMAC). The MUA:PDADMAC generates a strong blocking layer to the electron-transfer reaction involving the ferri/ferrocyanide redox couple. This blocking behaviour is not significantly affected upon adsorption of GO. However, adsorption of a sub-monolayer of rGO decreases the charge-transfer resistance by more than two orders of magnitude. Analysis of cyclic voltammograms and impedance spectra suggests that electron transfer in rGO assemblies is mediated by occupied states located just below the redox Fermi energy of the probe. These findings are discussed in the context of on-going controversies regarding the electrochemical reactivity of sp2-carbon basal planes.
Co-reporter:María G. Montes de Oca ; Daniela Plana ; Verónica Celorrio ; María J. Lazaro ;David J. Fermín
The Journal of Physical Chemistry C 2012 Volume 116(Issue 1) pp:692-699
Publication Date(Web):December 8, 2011
DOI:10.1021/jp208998j
The oxidations of carbon monoxide and formic acid at ultrathin Pd layers grown on Au nanoparticles were studied as a function of Pd thickness. Pd shells with thickness between 1 and 10 nm were grown on 19 nm Au nanoparticles by chemical reduction of H2PdCl4 with ascorbic acid. High-resolution transmission electron microscopy and X-ray diffraction confirm the core–shell configuration of the nanostructures. While the synthesis of pure Pd nanostructures led to a rather amorphous material, Pd nanoshells exhibited a polycrystalline structure confirming that Au nanostructures act as templates for Pd growth. Three-dimensional assemblies of nanoparticles were generated by alternate electrostatic layer-by-layer adsorption steps, involving poly-l-lysine and colloidal dispersions. Electrochemical studies in H2SO4 containing electrolyte solution demonstrate that CO coverage and anodic stripping potential are affected by the thickness of Pd nanoshells. In addition, the faradaic current density associated with HCOOH oxidation significantly increases with increasing Pd thickness. The thickness-dependent reactivity of Pd nanoshells is discussed in terms of lattice strain relaxation.
Co-reporter:V. Celorrio, M. G. Montes de Oca, D. Plana, R. Moliner, M. J. Lázaro, and D. J. Fermín
The Journal of Physical Chemistry C 2012 Volume 116(Issue 10) pp:6275-6282
Publication Date(Web):February 9, 2012
DOI:10.1021/jp211747a
The role of particle-substrate interactions on the reactivity of bimetallic nanostructures is investigated in the case of Au–Pd core–shell nanoparticles supported on Vulcan XC-72R (Vulcan). Core–shell nanostructures (CS) featuring 19 nm Au cores and Pd shells with thicknesses between ca. 1 and 10 nm were synthesized by controlled colloidal methods and subsequently incorporated in the carbon support. X-ray diffraction, energy dispersive X-ray analysis, and high resolution transmission electron microscopy confirmed the CS nature of the nanostructures, which remain unaffected upon incorporation onto the carbon matrix. Their electrochemical properties toward CO and HCOOH electro-oxidation were studied, using cyclic voltammetry and chronoamperometry. The results show that the CO stripping potential becomes independent of the average Pd lattice strain in the case of Vulcan supported CS. This behavior is significantly different to the trend observed in CS assemblies at In-doped SnO2 electrodes. Formic acid oxidation is also strongly affected not only by the thickness of the Pd nanoshell but also by the support. These reactivity trends are discussed in terms of strain (geometric) effects, CS crystalline structure, and substrate effects on the onset potential for the formation of oxygenated species at the catalyst surface.
Co-reporter:Amy Moore, Verónica Celorrio, María Montes de Oca, Daniela Plana, Wiphada Hongthani, María J. Lázaro and David J. Fermín
Chemical Communications 2011 vol. 47(Issue 27) pp:7656-7658
Publication Date(Web):07 Jun 2011
DOI:10.1039/C1CC12387D
The suitability of insulating highly crystalline diamond particles as support for Pd based electrocatalysts is explored for the first time by evaluating the electrochemical stripping of CO and oxidation of formic acid in acid solutions.
Co-reporter:Gabriela P. Kissling, David O. Miles and David J. Fermín
Physical Chemistry Chemical Physics 2011 vol. 13(Issue 48) pp:21175-21185
Publication Date(Web):05 Oct 2011
DOI:10.1039/C1CP21996K
Electron transfer processes mediated by nanostructured materials assembled at electrode surfaces underpin fundamental processes in novel electrochemical sensors, light energy conversion systems and molecular electronics. Functionalisation of electrode surfaces with hierarchical architectures incorporating self-assembling molecular systems and materials, such as metal nanostructures, quantum dots, carbon nanotubes, graphene or biomolecules have been intensively studied over the last 20 years. Important steps have been made towards the rationalisation of the charge transfer dynamics from redox species in solution across molecular self-assembling systems to electrode surfaces. For instance, a unified picture has emerged describing the factors which determine the rate constant for electron transfer processes across rigid self-assembling molecular barriers. An increasing bulk of evidence has recently shown that the incorporation of nanomaterials into self-assembling monolayers leads to an entirely different electrochemical behaviour. This perspective rationalises some of the key observations associated with nanoparticle mediated charge transfer, such as the apparent distance independent charge transfer resistance observed for redox species in solution. This behaviour only manifests itself clearly in the case where the probability of direct charge transfer from the redox probe to the electrode is strongly attenuated by self-assembling molecular barriers. Here we will highlight specific issues concerning self-assembled monolayers as blocking barriers prior to discussing the effect of nanoparticles on the electrochemical response of the system. Selected examples will provide conclusive evidence that the extent of charge transfer mediation is determined by the overlap between the density of states of the nanostructures and the energy levels of redox species in solution. Only in the case where a strong overlap exists between the energy levels of the two components, the nanostructures behave as “electron launchers”, allowing efficient charge transfer across insulating molecular layers.
Co-reporter:M. G. Montes de Oca ; H. Kumarakuru ; D. Cherns ;D. J. Fermín
The Journal of Physical Chemistry C 2011 Volume 115(Issue 21) pp:10489-10496
Publication Date(Web):May 5, 2011
DOI:10.1021/jp2021209
Electrochemically induced hydrogen adsorption at Au–Pd core–shell (CS) nanostructures was investigated as a function of the Pd lattice strain. CS nanoparticles with shell thicknesses between 1 and 10 nm were prepared by selective reduction of Pd on Au nanoparticles in aqueous solution. High-resolution transmission electron microscopy images and selected-area electron diffraction patterns confirmed the formation of epitaxial Pd films on the Au cores. Examination of diffraction patterns allowed a quantitative analysis of the lattice strain as a function of the shell thickness based on Matthews theory. This analysis showed that complete strain relaxation was not achieved even for Pd layers of 10 nm. Detailed electrochemical studies of two-dimensional assemblies with controlled particle number density provided valuable information not only on the H adsorption charge in acid solution but also on the average surface roughness of individual CS nanoparticles. These results show that apparent changes in the H adsorption charge density with increasing Pd thickness above 3 nm are mainly brought about by increasing shell roughness. Comparison with literature values suggests that these roughening effects are present in extended surfaces composed of a few atomic layers. The picture emerging from these results is that the H adsorption charge (coverage) is effectively independent of the Pd lattice strain. The implications of these studies with regard to established theoretical models for predicting the reactivity of epitaxial thin films are briefly discussed.
Co-reporter:Wiphada Hongthani, Neil A. Fox, and David J. Fermín
Langmuir 2011 Volume 27(Issue 8) pp:5112-5118
Publication Date(Web):March 15, 2011
DOI:10.1021/la1045833
The electrochemical properties of two-dimensional assemblies of 500 nm type Ib diamond particles are investigated as a function of their surface oxidation state. High Pressure High Temperature particles are sequentially exposed to a hot strong acid bath and to H2 plasma in order to generate oxygen (ODP) and hydrogen surface terminations (HDP). Changes in the surface composition following the chemical treatments are confirmed by FTIR. Electrophoretic mobility measurements show that the diamond particles exhibit a negative surface charge at pH above 7 independently of the surface termination. Oxidation in the acid bath and subsequent reduction in the H2 plasma only affects about 30% of the particle surface charge. The intrinsic negative charge allows the formation of 2D assemblies by electrostatic adsorption on poly(diallyldimethylammonium chloride) (PDADMAC) modified In-doped SnO2 electrodes (ITO). The particle number density in the assembly was controlled by the adsorption time up to a maximum coverage of ca. 40%. Cyclic voltammetry in the absence of redox species in solution show that the acid treatment effectively removes responses associated with sp2 carbon impurities, resulting in a potential independent capacitive signal. On the other hand, HDP assemblies are characterized by a charging process at a potential above 0.1 V vs Ag/AgCl. These responses are associated with hole-injection into the valence band edge which is shifted to approximately −4.75 eV vs vacuum upon hydrogenation. Information concerning the position of the valence band edge as well as hole number density at the HDP surface as a function of the applied potential are extracted from the electrochemical analysis.
Co-reporter:Gabriela P. Kissling ; Christa Bünzli ;David J. Fermín
Journal of the American Chemical Society 2010 Volume 132(Issue 47) pp:16855-16861
Publication Date(Web):November 4, 2010
DOI:10.1021/ja106149g
A novel approach to tuning electrochemical rectification using 2D assemblies of quantum dots (QDs) is presented. Asymmetric enhancement of the oxidation and reduction currents in the presence of the Fe(CN)63−/4− redox couple is observed upon adsorption of QDs at thiol-modified Au electrodes. The extent of the electrochemical rectification is dependent on the average QD size. A molecular blocking layer is generated by self-assembling 11-mercaptoundecanoic acid (MUA) and an ultrathin film of poly(diallyldimethylammonium chloride) (PDADMAC) on the electrode. The polycationic film allows the electrostatic adsorption of 3-mercaptopropionic acid (MPA)-stabilized CdTe QDs, generating 2D assemblies with approximately 0.4% coverage. The QD adsorption activates a fast charge transfer across the blocking layer in which the reduction process is more strongly enhanced than the oxidation reaction. The partial electrochemical rectification is rationalized in terms of the relative position of the valence (VB) and conduction band (CB) edges with respect to the redox Fermi energy (εredox). Quantitative analysis of the exchange current density obtained from electrochemical impedance spectroscopy demonstrates that the enhancement of charge transport across the molecular barrier is strongly dependent on the position of the QD valence band edge relative to εredox. The average electron tunneling rate constant through the QD assemblies is estimated on the basis of the Gerischer model for electron transfer.
Co-reporter:Christopher R. Bradbury, Lukas Kuster, David J. Fermín
Journal of Electroanalytical Chemistry 2010 Volume 646(1–2) pp:114-123
Publication Date(Web):15 July 2010
DOI:10.1016/j.jelechem.2010.04.015
The electrochemical reactivity of highly oriented pyrolytic graphite (HOPG) electrodes is systematically investigated upon sequential modification with ultrathin organic electrically insulating layers and two-dimensional arrays of Au nanoparticles. The modification of HOPG involved the electrochemical grafting of 4-carboxybenzenediazonium (4CB) followed by electrostatic adsorption of poly-l-lysine (PLL). The average thickness of the grafted films was 1.0 ± 0.5 nm as obtained from AFM analysis. Citrate stabilised Au nanoparticles with a diameter of 19 ± 3 nm were electrostatically adsorbed on PLL terminated film, generating a sub-monolayer with a particle number density Γnano = (6 ± 2) × 109 cm−2. The effect of each modification step on the dynamics of electron transfer involving the redox probes Fe(CN)63-/4-, Ru(NH3)63+ and hydroxymethyl-ferrocene (FcMeOH) was monitored by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The evolution of the phenomenological charge transfer resistance estimated from the impedance analysis revealed complex issues related to (i) the reactivity of HOPG basal planes, (ii) the role of electrostatic and hydrophobic interactions between redox probes and the ultrathin layers as well as (iii) the role of nanoparticles as electron transfer mediators. We conclude that the electrochemical reactivity of the basal plane is sensitive to the ionic and solvation environment of the redox probe.
Co-reporter:David J. Fermín, Frank Marken
Journal of Electroanalytical Chemistry 2010 Volume 646(1–2) pp:1-2
Publication Date(Web):15 July 2010
DOI:10.1016/j.jelechem.2010.06.017
Co-reporter:Mathias Schnippering, Alain Zahn, Shi-Xia Liu, Christian Leumann, Silvio Decurtins and David J. Fermín
Chemical Communications 2009 (Issue 37) pp:5552-5554
Publication Date(Web):26 Aug 2009
DOI:10.1039/B907847A
A new approach for labelling oligodeoxynucleotides with tetrathiafulvalene (TTF) units is presented, generating single and double strand structures with distinctive electrochemical signatures.
Co-reporter:Gabriela P. Kissling and David J. Fermín
Physical Chemistry Chemical Physics 2009 vol. 11(Issue 43) pp:10080-10086
Publication Date(Web):16 Sep 2009
DOI:10.1039/B910127F
The electrochemical injection of holes into the valence band of mercaptopropionic acid stabilised CdTe quantum dots in aqueous solution was investigated employing a glassy carbon rotating disc electrode. Analysis of the first exciton peak and the electrochemical responses as a function of the particle size was performed within the framework of the effective mass approximation and tight-binding models. It is demonstrated that the energy of the quantum dot band edges can be predicted from capacitance data of bulk semiconductor electrodes modified by the same stabilising groups as on the dots. In this paper we show that the thiol binding shifts the band edges of the CdTe particles by approximately 0.7 eV with respect to the theoretical value in the absence of stabilising groups. The results also revealed a significant dependence of the hole-injection rate on the electrochemical potential.
Co-reporter:Christopher R. Bradbury ; Jianjun Zhao ;David J. Fermín
The Journal of Physical Chemistry C 2008 Volume 112(Issue 27) pp:10153-10160
Publication Date(Web):June 17, 2008
DOI:10.1021/jp710165d
The electrochemical properties of Au electrodes sequentially modified by self-assembled monolayers (SAM) of carboxyl-terminated alkane thiols, ultrathin poly-l-lysine (PLL) film, and diluted monolayers of Au nanoparticles are investigated by electrochemical impedance spectroscopy (EIS). The phenomenological charge-transfer resistance (Rct) for the hexacyanoferrate redox couple at the equilibrium potential exhibited an exponential increase with increasing methylene units (x) in the SAM. The increase of Rct between x = 1 and 10 was described by a well-defined decay parameter β = 1.16 ± 0.04 per methylene unit. This behavior suggests that the kinetics of electron transfer is controlled by coherent electron tunneling across the carboxyl-terminated SAM. Adsorption of the PLL brings about an average 2.5 times decrease in Rct independent of x. The ultrathin PLL film (thickness less than 1 nm) induces an increase of the surface concentration of the redox couple without affecting the β value observed for the SAM-terminated electrodes. Diluted monolayers of Au nanoparticles with an average 19.2 ± 2.1 nm diameter generate significant changes in the dynamics of electron transfer. In contrast to the behavior in the absence of nanoparticles, a distance-independent Rct was observed for x > 5. Detailed analysis of the electrochemical responses as a function of the particle number density revealed that the rate-determining step is the charging of the nanoparticles by the redox species. It is concluded that the electronic communication between the nanoparticles and the electrode surface over distances as large as 13 Å originates from electron transport through the trapped redox probe. The several orders of magnitude changes of the apparent Rct upon nanoparticle adsorption further suggest that electron transport through the film does not occur via a classical hopping mechanism. A mechanism based on nonthermalized electron transport via the density of the redox probe at the Fermi energy (hot electron transport) is proposed to account for the experimental observations.
Co-reporter:Fei Li ; Ilenia Ciani ; Paolo Bertoncello ; Patrick R. Unwin ; Jianjun Zhao ; Christopher R. Bradbury
The Journal of Physical Chemistry C 2008 Volume 112(Issue 26) pp:9686-9694
Publication Date(Web):June 11, 2008
DOI:10.1021/jp8001228
The kinetics of the hydrogen evolution reaction (HER) catalyzed by two-dimensional assemblies of 13 nm diameter palladium (Pd) nanoparticles on mica substrates was investigated by scanning electrochemical microscopy (SECM). The assemblies were prepared by electrostatic adsorption of citrate-stabilized Pd nanoparticles on poly-l-lysine treated mica. Atomic force microscopy (AFM) studies of the adsorption process provided information on the nanoparticle number density as a function of the adsorption time. The HER kinetics was determined by examining SECM feedback approach curves, employing the methyl viologen (MV2+/+•) couple as the redox probe. With this configuration, the potential of the Pd nanoparticles is effectively determined by the local concentration ratio of the redox probe. The overpotential for proton reduction can be finely tuned by the concentration of the redox species, the size of the ultramicroelectrode (UME) tip, and the distance between the UME and the nanoparticle assembly. The SECM analysis allowed the mean exchange current density per Pd nanoparticle [j0(pH = 3) = (1.19 ± 0.08) × 10−6 A cm−2] to be evaluated. Significantly, the SECM methodology described is highly sensitive to the transfer coefficient of the HER, with a value of 0.5 providing the most satisfactory fit in the overpotential range investigated. It is concluded that the reactivity of the Pd nanoparticles is comparable to the bulk metal, which is consistent with the bulk-like electronic structure of Pd clusters of this dimension.
Co-reporter:Nessrin Kattan, Bo Hou, David J. Fermín, David Cherns
Applied Materials Today (November 2015) Volume 1(Issue 1) pp:
Publication Date(Web):1 November 2015
DOI:10.1016/j.apmt.2015.08.004
•Fingerprint electron diffraction patterns for kesterite/stannite CZTS.•Clear contrast with ZnS secondary phase.•Visualization of defects in CZTS in the transition from nano to microcrystal.•Observation of preferential crystal growth on {112} planes.A detailed analysis of the structure of Cu2ZnSnS4 (CZTS) nanocrystals synthesized by hot-injection in the presence of oleylamine is provided employing high resolution TEM, selected area electron diffraction (SAED) and convergent beam electron diffraction (CBED). The nanostructures were investigated as-grown and after vacuum thermal treatment at 550 °C. As-grown materials consisted of polycrystalline particles with an average size of 7 ± 3 nm, which grow an average size of 53 ± 13 nm after the vacuum annealing step. This thermal treatment allows investigating the initial stages of high quality film growth required in photovoltaic devices. Sets of SAED and CBED patterns, where individual crystals after annealing were viewed down different prominent zone axes, enabled us to reveal the presence of weak reflections due to cation ordering, and confirm a tetragonal unit cell consistent with either the kesterite or stannite structure. We demonstrate how these approaches enable to distinguish CZTS from secondary phases such as ZnS. Structure defects of partially annealed CZTS crystals were also investigated using bright and dark field images taken in 2-beam diffraction conditions as well as by high resolution lattice imaging. The material exhibited dislocations, along with lamellar twins and stacking faults characterized by local hexagonal structure on {112} planes. High resolution TEM images showed preferential growth on {112} planes during vacuum annealing, which is consistent with X-ray diffraction patterns. These studies provide key information on nanoscale crystal defects which may have important consequence on the performance of CZTS photovoltaic devices.Graphical abstractDownload high-res image (66KB)Download full-size image
Co-reporter:Gabriela P. Kissling and David J. Fermín
Physical Chemistry Chemical Physics 2009 - vol. 11(Issue 43) pp:NaN10086-10086
Publication Date(Web):2009/09/16
DOI:10.1039/B910127F
The electrochemical injection of holes into the valence band of mercaptopropionic acid stabilised CdTe quantum dots in aqueous solution was investigated employing a glassy carbon rotating disc electrode. Analysis of the first exciton peak and the electrochemical responses as a function of the particle size was performed within the framework of the effective mass approximation and tight-binding models. It is demonstrated that the energy of the quantum dot band edges can be predicted from capacitance data of bulk semiconductor electrodes modified by the same stabilising groups as on the dots. In this paper we show that the thiol binding shifts the band edges of the CdTe particles by approximately 0.7 eV with respect to the theoretical value in the absence of stabilising groups. The results also revealed a significant dependence of the hole-injection rate on the electrochemical potential.
Co-reporter:Mathias Schnippering, Alain Zahn, Shi-Xia Liu, Christian Leumann, Silvio Decurtins and David J. Fermín
Chemical Communications 2009(Issue 37) pp:NaN5554-5554
Publication Date(Web):2009/08/26
DOI:10.1039/B907847A
A new approach for labelling oligodeoxynucleotides with tetrathiafulvalene (TTF) units is presented, generating single and double strand structures with distinctive electrochemical signatures.
Co-reporter:Gabriela P. Kissling, David O. Miles and David J. Fermín
Physical Chemistry Chemical Physics 2011 - vol. 13(Issue 48) pp:NaN21185-21185
Publication Date(Web):2011/10/05
DOI:10.1039/C1CP21996K
Electron transfer processes mediated by nanostructured materials assembled at electrode surfaces underpin fundamental processes in novel electrochemical sensors, light energy conversion systems and molecular electronics. Functionalisation of electrode surfaces with hierarchical architectures incorporating self-assembling molecular systems and materials, such as metal nanostructures, quantum dots, carbon nanotubes, graphene or biomolecules have been intensively studied over the last 20 years. Important steps have been made towards the rationalisation of the charge transfer dynamics from redox species in solution across molecular self-assembling systems to electrode surfaces. For instance, a unified picture has emerged describing the factors which determine the rate constant for electron transfer processes across rigid self-assembling molecular barriers. An increasing bulk of evidence has recently shown that the incorporation of nanomaterials into self-assembling monolayers leads to an entirely different electrochemical behaviour. This perspective rationalises some of the key observations associated with nanoparticle mediated charge transfer, such as the apparent distance independent charge transfer resistance observed for redox species in solution. This behaviour only manifests itself clearly in the case where the probability of direct charge transfer from the redox probe to the electrode is strongly attenuated by self-assembling molecular barriers. Here we will highlight specific issues concerning self-assembled monolayers as blocking barriers prior to discussing the effect of nanoparticles on the electrochemical response of the system. Selected examples will provide conclusive evidence that the extent of charge transfer mediation is determined by the overlap between the density of states of the nanostructures and the energy levels of redox species in solution. Only in the case where a strong overlap exists between the energy levels of the two components, the nanostructures behave as “electron launchers”, allowing efficient charge transfer across insulating molecular layers.
Co-reporter:Amy Moore, Verónica Celorrio, María Montes de Oca, Daniela Plana, Wiphada Hongthani, María J. Lázaro and David J. Fermín
Chemical Communications 2011 - vol. 47(Issue 27) pp:NaN7658-7658
Publication Date(Web):2011/06/07
DOI:10.1039/C1CC12387D
The suitability of insulating highly crystalline diamond particles as support for Pd based electrocatalysts is explored for the first time by evaluating the electrochemical stripping of CO and oxidation of formic acid in acid solutions.
Co-reporter:Daniela Plana, Jonathan Flórez-Montaño, Veronica Celorrio, Elena Pastor and David J. Fermín
Chemical Communications 2013 - vol. 49(Issue 93) pp:NaN10964-10964
Publication Date(Web):2013/10/14
DOI:10.1039/C3CC46543H
The faradaic efficiency of CO2 electroreduction is significantly affected by the thickness of Pd nanoshells on Au cores. The ratio of hydrogen evolution to CO2 reduction was determined by differential electrochemical mass spectrometry. Decreasing the Pd shell thickness from 10 to 1 nm leads to a twofold increase in faradaic efficiency.
Co-reporter:Bo Hou, David Benito-Alifonso, Richard Webster, David Cherns, M. Carmen Galan and David J. Fermín
Journal of Materials Chemistry A 2014 - vol. 2(Issue 19) pp:NaN6886-6886
Publication Date(Web):2014/02/19
DOI:10.1039/C4TA00285G
The replacement of phosphine containing compounds in the synthesis of II–VI quantum dots (QDs) via the ‘hot-injection’ method has received considerable attention in recent years, in particular toward scaling-up production. A key bottleneck in current approaches is the poor solubility of elemental Se in solvents such as 1-octadecene (1-ODE) or oleylamine requiring a heating temperature of 200 °C for several hours, or the introduction of additives such as alkylthiols or NaBH4, or replacement by SeO2. In the present work, we elucidate the fundamental steps in the dissolution of elemental Se in 1-ODE and oleylamine with the view of facilitating the large-scale synthesis of CdSe QDs. The main organoselenium species generated during the solubilisation of elemental Se in 1-ODE and oleylamine were identified by 1D and 2D NMR spectroscopy (1H, 13C and 77Se). Experimental evidence suggests that the rate determining step is the formation of Se radicals, via homolytic cleavage of the Se–Se bond, that attack the allylic proton in 1-ODE and oleylamine. Plausible reaction pathways in both systems are proposed. Finally, we demonstrate that the radical-mediated solubilisation of Se can be significantly accelerated by the addition of azobisisobutyronitrile (AIBN), a common radical initiator used in the polymer industry. In this way, a highly concentrated Se precursor was prepared. The “hot-injection” of the Se precursor into CdO containing 1-ODE solution leads to the formation of highly luminescent CdSe QDs with a well-defined cubic structure.
Co-reporter:D. Plana, K. A. Bradley, D. Tiwari and D. J. Fermín
Physical Chemistry Chemical Physics 2016 - vol. 18(Issue 18) pp:NaN12433-12433
Publication Date(Web):2016/04/15
DOI:10.1039/C6CP02231F
The efficiency of photoelectrochemical reactions is conventionally defined in terms of the ratio between the current responses arising from the collection of carriers at electrical contacts and the incident photon flux at a given wavelength, i.e. the incident-photon-to-current-efficiency (IPCE). IPCE values are determined by a variety of factors such as the absorption constant of the active layer, bulk and surface recombination of photogenerated carriers, as well as their characteristic diffusion length. These parameters are particularly crucial in nanostructured photoelectrodes, which commonly display low carrier mobility. In this article, we examine the photoelectrochemical responses of a mesoporous TiO2 film in which the IPCE is enhanced by fast extraction of carriers via chemical reactions. TiO2 films are spontaneously formed by destabilisation of colloidal particles at the polarisable interface between two immiscible electrolyte solutions. The photocurrent arises from hole-transfer to redox species confined to the organic electrolyte, which is coupled to the transfer of electrons to oxygen in the aqueous electrolyte. The dynamic photocurrent responses demonstrate that no coupled ion transfer is involved in the process. The interplay of different interfacial length scales, molecularly sharp liquid/liquid boundary and mesoporous TiO2 film, promotes efficiencies above 75% (without correction for reflection losses). This is a significant step change in values reported for these interfaces (below 1%), which are usually limited to sub-monolayer coverage of photoactive molecular or nanoscopic materials.
Co-reporter:Verónica Celorrio, Laura Calvillo, Ellie Dann, Gaetano Granozzi, Ainara Aguadero, Denis Kramer, Andrea E. Russell and David J. Fermín
Catalysis Science & Technology (2011-Present) 2016 - vol. 6(Issue 19) pp:NaN7238-7238
Publication Date(Web):2016/07/13
DOI:10.1039/C6CY01105E
The mean activity of surface Mn sites at LaxCa1−xMnO3 nanostructures towards the oxygen reduction reaction (ORR) in alkaline solution is assessed as a function of the oxide composition. Highly active oxide nanoparticles were synthesised by an ionic liquid-based route, yielding phase-pure nanoparticles, across the entire range of compositions, with sizes between 20 and 35 nm. The bulk vs. surface composition and structure are investigated by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES). These techniques allow quantification of not only changes in the mean oxidation state of Mn as a function of x, but also the extent of A-site surface segregation. Both trends manifest themselves in the electrochemical responses associated with surface Mn sites in 0.1 M KOH solution. The characteristic redox signatures of Mn sites are used to estimate their effective surface number density. This parameter allows comparing, for the first time, the mean electrocatalytic activity of surface Mn sites as a function of the LaxCa1−xMnO3 composition. The ensemble of experimental data provides a consistent picture in which increasing electron density at the Mn sites leads to an increase in the ORR activity. We also demonstrate that normalisation of electrochemical activity by mass or specific surface area may result in inaccurate structure–activity correlations.
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