IN PRESS - Selected, just-accepted articles

  Label-free immunodetection of α-synuclein by using a microfluidics coplanar electrolyte-gated organic field-effect transistor

Simona Ricci, Stefano Casalini, Vitaliy Parkula, Meenu Selvaraj, Gulseren Deniz Saygin, Pierpaolo Greco, Fabio Biscarini, Marta Mas-Torrent

Biosensors and Bioelectronics

IF=10.257

The aggregation of α-synuclein is a critical event in the pathogenesis of neurological diseases, such as Parkinson or Alzheimer. Here, we present a label-free sensor based on an Electrolyte-Gated Organic Field-Effect Transistor (EGOFET) integrated with microfluidics that allows us to detect amounts of α-synuclein in the range from 0.25 pM to 25 nM. The lower limit of detection (LOD) measures the potential of our integrated device as a tool for prognostics and diagnostics. In our device, the gate electrode is the effective sensing element as it is functionalized with anti-(α-synuclein) antibodies using a dual strategy: i) an amino-terminated self-assembled monolayer activated by glutaraldehyde, and ii) the His-tagged recombinant protein G. In both approaches, comparable sensitivity values were achieved, featuring very low LOD values at the sub-pM level. The microfluidics engineering is central to achieve a controlled functionalization of the gate electrode and avoid contamination or physisorption on the organic semiconductor. The demonstrated sensing architecture, being a disposable stand-alone chip, can be operated as a point-of-care test, but also it might represent a promising label-free tool to explore in-vitro protein aggregation that takes place during the progression of neurodegenerative illnesses.

  Insights into the mechanism of coreactant electrochemiluminescence facilitating enhanced bioanalytical performance

Alessandra Zanut, Andrea Fiorani, Sofia Canola, Toshiro Saito, Nicole Ziebart, Stefania Rapino, Sara Rebeccani, Antonio Barbon, Takashi Irie, Hans-Peter Josel, Fabrizia Negri, Massimo Marcaccio, Michaela Windfuhr, Kyoko Imai, Giovanni Valenti & Francesco Paolucci

Nature Communications volume 11, Article number: 2668 (2020)

https://doi.org/10.1038/s41467-020-16476-2

Electrochemiluminescence (ECL) is a powerful transduction technique with a leading role in the biosensing field due to its high sensitivity and low background signal. Although the intrinsic analytical strength of ECL depends critically on the overall efficiency of the mechanisms of its generation, studies aimed at enhancing the ECL signal have mostly focused on the investigation of materials, either luminophores or coreactants, while fundamental mechanistic studies are relatively scarce. Here, we discover an unexpected but highly efficient mechanistic path for ECL generation close to the electrode surface (signal enhancement, 128%) using an innovative combination of ECL imaging techniques and electrochemical mapping of radical generation. Our findings, which are also supported by quantum chemical calculations and spin trapping methods, led to the identification of a family of alternative branched amine coreactants, which raises the analytical strength of ECL well beyond that of present state-of-the-art immunoassays, thus creating potential ECL applications in ultrasensitive bioanalysis.

  Ligand-free ZnS nanoparticles: as easy and green as it gets

Nicola Dengo, Andrea Faresin, Tommaso Carofiglio, Michele Maggini*, Longfei Wu, Jan P. Hofmann, Emiel J. M. Hensen, Paolo Dolcet, and Silvia Gross*

Communication

ChemComm, Just Accepted Manuscript

I. F.: 6.164

The controlled nucleation and crystallization of small pure sphalerite ZnS nanoparticles was achieved under batch and continuous flow conditions at low temperature, in water and without the use of any stabilizing ligand. The obtained nanoparticles displayed a narrow size distribution and high specific surface area. Moreover, the synthesis was suitable to directly obtain stable water-based suspensions and the products were found to be active photocatalysts for the hydrogen evolution reaction.

  Establishing Reactivity Descriptors for Platinum Group Metal (PGM)-free Fe-N-C Catalysts for PEM Fuel Cells

Mathias Primbs, Yanyan Sun, Aaron Roy, Daniel Malko, Asad Mehmood, Moulay-Tahar Sougrati, Pierre-Yves Blanchard, Gaetano GranozziTomasz KosmalaGiorgia Daniel, Plamen Atanassov, Jonathan Sharman, Christian Durante, Anthony Kucernak, Deborah Jones, Frédéric Jaouen, Peter Strasser

Energy and Environmental Science., 2020, (IF: 33.250)

DOI: 10.1039/D0EE01013H

We report a comprehensive analysis of the catalytic oxygen reduction reaction (ORR) reactivity of four of today’s most active benchmark platinum group metal-free (PGM-free) iron/nitrogen doped carbon electrocatalysts (Fe-N-Cs). Our analysis reaches far beyond previous such attempts in linking kinetic performance metrics, such as electrocatalytic mass-based and surface area-based catalytic activity with previously elusive kinetic metrics such as the active metal site density (SD) and the catalytic turnover frequency (TOF). Kinetic ORR activities, SD and TOF values were evaluated using in-situ electrochemical NO2- reduction as well as an ex-situ gaseous CO cryo chemisorption. Experimental ex-situ and in-situ Fe surface site densities displayed remarkable quantitative congruence. Plots of SD versus TOF (“reactivity maps”) are utilized as new analytical tools to deconvolute ORR reactivities and thus enabling rational catalyst developments. A microporous catalyst showed large SD values paired with low TOF, while mesoporous catalysts displayed the opposite. Trends in Fe surface site density were linked to molecular nitrogen and Fe moieties (D1 and D2 from 57Fe Mössbauer spectroscopy), from which pore locations of catalytically active D1 and D2 sites were established. This cross-laboratory analysis, its employed experimental practices and analytical methodologies are expected to serve as a widely accepted reference for future, knowledge-based research into improved PGM-free fuel cell cathode catalysts.

  Stable, Active and Methanol Tolerant PGM-free Surface in Acidic Medium: Electron Tunneling at Play in Pt/FeNC Hybrid Catalysts for Direct Methanol Fuel Cell Cathode

Tomasz Kosmala, Nicolas Bibent, Moulay Tahar Sougrati, Goran Dražić, Stefano Agnoli, Frédéric Jaouen and Gaetano Granozzi

ACS Catalysis., 2020, (IF: 12.221)

doi.org/10.1021/acscatal.0c01288

PGM-free catalysts have high initial activity for O2 reduction reaction, but suffer from low stability in acid medium in PEMFC and DMFC. Here, we shed light on the atomic-scale structure of hybrid Pt/FeNC catalysts (1-2 wt% of Pt), revealing by STEM and EDXS the presence of Pt@FeOx particles. The absence of exposed Pt on the surface is confirmed by the suppression of methanol oxidation reaction and CO stripping experiments. The promising application of such Pt/FeNC catalysts, comprising FeNx sites and Pt@FeOx particles, is demonstrated at the cathode of DMFC. To gain fundamental understanding on the stability in acid medium and on the intrinsic ORR activity of Pt@FeOx, we constructed model surfaces by depositing FeOx films with controlled thickness (from 1.0 to 6.4 nm), fully covering the Pt(111) surface, which resulted stable in acid medium in the potential range 0.45 – 1.05 V vs. RHE. The specific ORR activity of Fe2O3/Pt(111) increases exponentially with decreasing overlayer thickness, which is explained by the tunneling of Pt electrons through Fe2O3. This special phenomenon sheds light onto recently reported excellent durability of Pt/FeNC composites in PEMFC and identify a promising core@shell strategy leading to stable PGM-free surfaces in acid medium, and tolerant to methanol.

  Copper Vanadate Nanobelts as Anodes for Photoelectrochemical Water Splitting: Influence of CoOx Overlayers on Functional Performances

Leonardo Girardi, Gian Andrea Rizzi, Lorenzo Bigiani, Davide Barreca, Chiara Maccato, Carla Marega and Gaetano Granozzi

ACS Appl. Mater. Interfaces, Just Accepted Manuscript

(IF: 8.456)

DOI: 10.1021/acsami.0c0691

The design and development of environmentally friendly and robust anodes for photoelectrochemical (PEC) water splitting plays a critical role for the efficient conversion of radiant energy into hydrogen fuel. In this regard, quasi 1D copper vanadates (CuV2O6) were grown on conductive substrates by a hydrothermal procedure and processed for use as anodes in PEC cells, with particular attention on the role exerted by cobalt oxide (CoOx) overlayers deposited by radio frequency (RF)-Sputtering. The target materials were characterized in detail by a multi-technique approach, with the aim at elucidating the interplay between their structure, composition, morphology and the resulting activity as photoanodes. Functional tests were performed by standard electrochemical techniques like linear sweep voltammetry, impedance spectroscopy and by the less conventional intensity modulated photocurrent spectroscopy, yielding an important insight into the material PEC properties. The obtained results highlight that, despite the supposedly favourable band alignment between CuV2O6 and Co3O4 did not yield a net current density increase, cobalt oxide-functionalized anodesafforded a remarkable durability enhancement, an important pre-requisite for their eventual real-world applications. The concurrent phenomena accounting for the observed behavior are presented and discussed in relation to material physico-chemical properties.

  Boron oxynitride two-colours fluorescent dots and their incorporation in a hybrid organic-inorganic film

Junkai Ren, Luca Malfatti, Stefano Enzo, Carlo Maria Carbonaro, Laura Calvillo, Gaetano Granozzi, Plinio Innocenzi

Journal of Colloid and Interface Science 560 (2020) 398–406, (IF: 6.361)

doi.org/10.1016/j.jcis.2019.10.020

Bottom-up synthesis of fluorescent boron-nitride based dots is a challenging task because an accurate design of the structure-properties relationship is, in general, difficult to achieve. Incorporation of the dots into a solid-state matrix is also another important target to develop light-emitting devices.

Two-colour fluorescent boron oxynitride nanodots have been obtained by a bottom-up synthesis route and incorporated into a hybrid organic-inorganic film. A combination of different analytical techniques such as XPS, XRD, TEM, UV–Vis, TGA-DTA and fluorescence has been used to characterise the structure, composition and properties of the boron oxynitride dots. The presence of defects in the boron oxynitride structure is the source of the two-colour fluorescence. The BN dots thermal stability is limited to around 100 °C; higher temperatures induce condensation of the structure, which leads to a lower emission. Upon incorporation into a hybrid organic-inorganic film deposited by spin-coating, the boron oxynitride dots maintain their fluorescence and have shown to be highly compatible with the sol-gel chemistry.

  Tuning on and off chemical- and photo-activity of exfoliated MoSe2 nanosheets through morphologically selective “soft” covalent functionalization with porphyrins

Matias Blanco, Marco Lunardon, M. Bortoli, Dario Mosconi, Leonardo Girardi, Laura Orian, Stefano Agnoli and Gaetano Granozzi

J. Mat. Chem. A  2020, (IF: 10.733)

DOI: 10.1039/D0TA03302B

The covalent functionalization of 2D transition metal dichalcogenides (TMDCs) with organic molecules exploiting the thiol conjugation approach is still a very controversial topic. We have synthesized thiolated tetraphenyl porphyrins, with and without hydroxyl groups in the phenyl substituents, which have been covalently attached to chemically exfoliated MoSe2 (ce-MoSe2) nanosheets. XPS and FTIR have revealed the formation of sulfo-selenide bridges, specifically on the edges of the ce-MoSe2, as confirmed by first principle calculations. The TMDCs’ electrocatalytic activity in the hydrogen evolution reaction (HER) has been investigated after conjugation with the organic molecules. The HER activity is suppressed or enhanced according to the presence of mildly acid hydroxyls groups in the attached molecules, since they provide a local proton relay boosting the production of H2, especially in mildly acidic conditions (pH=4.3). Moreover, the well-known light-harvesting properties of porphyrins have been exploited to improve significantly the light-assisted HER activity. Due to the formation of a type II heterojunction or Schottky contact between the molecules and the 2H and 1T MoSe2 nanosheets respectively, the hybrid materials show an improvement of the HER onset potential under illumination compared to the pristine material, without activity loss for more than 16 h.

  Low-temperature wet chemistry synthetic approaches towards ferrites

Stefano Diodati, Richard Walton, Simone Mascotto, Silvia Gross*

Inorg. Chem. Front., Invited review, Just Accepted Manuscript

I. F.: 5.934

Ferrites are a broad class of oxides containing iron which in general include spinel ferrites MFe2O4, perovskites, ilmenite (FeTiO3) and hexagonal ferrites (or hexaferrites). These materials have a wide array of applications owing to their diverse properties. In this review, the different low-temperature (<200°C), wet chemistry approaches employed in recent years for the synthesis of ferrites are summarised, categorised, classified and examined. This work collects and reviews recent contributions concerning different wet-chemistry routes, having as common factor the use of low temperature, for the synthesis of different classes of ferrites, focussing largely on materials belonging to the spinel, perovskite and hexaferrite families. The discussion of the different examples reported illustrates that this relevant family of functional materials can be also addressed by relatively environmentally friendly and, in many cases, simple to implement solution- or suspension-based synthesis approaches.

  Low-temperature solution crystallisation of nanostructured oxides and thin films

Iñigo Bretos, Stefano Diodati, Ricardo Jiménez Riobóo, Francesca Tajoli, Jesús Ricote, Giulia Bragaggia, Marina Franca, Maria Lourdes Calzada*, Silvia Gross*

Review in press in Chemistry – A European Journal, 2020

I. F.: 5.160

The increasing demand of high-performance crystalline oxide nanomaterials for applications in current and emergent technologies is pushing research on alternative and sustainable synthesis processes, ideally complying with the principles of Green Chemistry. In this framework, wet-chemistry approaches are particularly promising, since they are typically based on soft synthesis processes, in terms of temperature and pressure, or use water and/or non-toxic alcohols as solvents. Solution methods are low-cost and simple approaches to the synthesis of nanomaterials but, to achieve sustainable, environmentally friendly and competitive manufacturing, they should address an additional target, i.e. the reduction of process energy consumption. This is one of the paradigms of Green Chemistry, where low temperatures and mild conditions of synthesis are specifically pursued. Using low temperatures for the preparation of a nanomaterial or of a nanostructured thin film does not necessarily imply sacrificing crystallinity and compositional purity of the obtained sample. Indeed, low temperatures actually afford tighter control on size and size distribution of the obtained nanostructures and of the microstructure and heterostructure of the resulting films.
In this context, the present progress report provides a brief overview of low-temperature solution methods reported in the literature to attain crystalline oxide nanostructures and thin films.

  An atmospheric pressure plasma jet to tune the bioactive peptide coupling to polycaprolactone electrospun layers

Alessandro Maffei, Niccoló  Michieli, Paola Brun, Annj Zaunera, Alessandro Zaggia, Martina Roso, Boris Kalinic, Emanuele Verga Falzacappa, Paolo Scopece, Silvia Gross, Monica Dettin, Alessandro Patelli

Applied Surface Science, 507 (2020) 144713

I. F.: 5.155

The surface chemistry of scaffolds for tissue regeneration can guide cells growth. In this study, we present a novel method to functionalize electrospun Polycaprolactone (PCL) scaffolds allowing the tuning of biomolecule superficial concentration by varying only one process parameter. The method is based on the deposition of NH2 functional groups starting form (3-Aminopropyl) triethoxysilane (APTES) as precursor by a novel Atmospheric Pressure Plasma Jet (APPJ) and by a successive selective covalent linking of these amines with a synthetic Human Vitronectin adhesive cue (HVP). The addition, in the peptide C-terminus, of an aldehyde group ensures the selective ligation by alkylimino-de-oxo-bisubstitution between the primary amine and HVP. By this method, we managed to alter the HVP surface concentration just varying the deposition time of the plasma process; this resulted in different surface coverage of the plasma coating, which in turn led to diverse amount of linked HVP. Coating stability, morphology and coverage was assessed by infrared and photo-electron spectroscopies and by electron microscopy. As a function of the coverage a variation on peptide concentration was revealed by Total Nitrogen method and confirmed by biological assays, which demonstrated an increase of human osteoblasts viability as a function of peptide concentration.

  The role of the synthetic pathways on properties of Ag2S nanoparticles for photothermal applications

Jessica Munaro, Paolo Dolcet, Silvia Nappini, Elena Magnano, Nicola Dengo, Giacomo Lucchini, Adolfo Speghini, Silvia Gross

Applied Surface Science, 2020, 145856, doi.org/10.1016/j.apsusc.2020.145856

I. F.: 5.155

Two different synthetic pathways, namely the miniemulsion and hydrothermal approaches, were investigated for the preparation of silver sulfide nanoparticles, also doped with lanthanide ions. It was demonstrated that the synthesis and the choice of the precursors play a fundamental role in determining the crystalline purity, the size, the stability and the functional properties of the Ag2S nanostructures. The miniemulsion approach leads to the formation of the targeted structures already at room temperature, but the hydrothermal approach yields particles with increased purity and long-term stability against oxidation. Synchrotron XPS performed with different penetration depths evidences the presence of a stable core of the crystallites composed uniquely of Ag2S and an external shell where residual elemental S demonstrates the first stage of oxidation due to atmospheric exposure. The samples prepared via hydrothermal synthesis also show a high optical density (0.377) and a good laser-to-heat conversion efficiency (29 %).

  Surface Engineering of Chemically Exfoliated MoS2 in a “Click”: How to Generate Versatile Multifunctional Transition Metal Dichalcogenides-Based Platforms

Giulia Tuci, Dario Mosconi, Andrea Rossin, Lapo Luconi, Stefano Agnoli, Marcello Righetto, Cuong Pham-Huu, Housseinou Ba, Stefano Cicchi, Gaetano Granozzi,* and Giuliano Giambastiani*

Chemistry of Materials (IF= 9.890)

DOI: 10.1021/acs.chemmater.8b03663

Accepted on 11 October 2018

The interest for transition metal dichalcogenides (TMDs) as two-dimensional (2D) analogues of graphene is steadily growing along with the need of efficient and easy tunable protocols for their surface functionalization. This latter aspect holds a key role in the widespread application of TMDs in various technological fields and it represents the missing step to bridge the gap between the more popular Csp2-based networks and their inorganic counterparts. Although significant steps forward have already been made in the field of TMDs functionalization (particularly for MoS2), a rational approach to their surface engineering for the generation of 2D organic-inorganic hybrids capable to accommodate various molecules featured by orthogonal groups has not been reported yet. The paper paves the way towards a new frontier for “click" chemistry in material science. It describes the post-synthetic modification (PSM) of covalently decorated MoS2 nanosheets with phenylazido pendant arms and the successful application of CuAAC chemistry (copper-mediated azide-alkyne cycloaddition) towards the generation of highly homo- and hetero-decorated MoS2 platforms. This contribution goes beyond the proof of evidence of the chemical grafting of organic groups to the surface of exfoliated MoS2 flakes through covalent C-S bonds. It also demonstrates the versatility of the hybrid samples to undergo post-synthetic modifications thus imparting multimodality to these 2D materials.

  Non-covalent integration of a bio-inspired Ni catalyst to graphene acid for reversible electrocatalytic hydrogen oxidation

Bertrand Reuillard, Matías Blanco, Laura Calvillo, Nathan Coutard, Ahmed Ghedjatti, Pascale Chenevier, Stefano Agnoli, Michal Otyepka, Gaetano Granozzi, and Vincent Artero

ACS Appl. Mater. Interfaces, Just Accepted Manuscript •, (IF: 8.456)

DOI: 10.1021/acsami.9b18922

Efficient heterogeneous catalysis of hydrogen oxidation reaction (HOR) by platinum group metal (PGM)-free catalysts in proton-exchange membrane (PEM) fuel cells represents a significant challenge toward the development of a sustainable hydrogen economy. Here, we show that graphene acid (GA) can be used as an electrode scaffold for the noncovalent immobilization of a bioinspired nickel bis-diphosphine HOR catalyst. The highly functionalized structure of this material and optimization of the electrode-catalyst assembly sets new benchmark electrocatalytic performances for heterogeneous molecular HOR, with current densities above 30 mA cm–2 at 0.4 V versus reversible hydrogen electrode in acidic aqueous conditions and at room temperature. This study also shows the great potential of GA for catalyst loading improvement and porosity management within nanostructured electrodes toward achieving high current densities with a noble-metal free molecular catalyst.

  Formation of metal-organic ligand complexes affects solubility of metals in airborne particles at an urban site in the Po Valley

Andrea Tapparo, Valerio Di Marco, Denis Badocco, Sara D’Aronco, Lidia Soldà, Paolo Pastore, Brendan M. Mahon, Markus Kalberer, and Chiara Giorio

Chemosphere (IF = 5.108)

DOI: 10.1016/j.chemosphere.2019.125025

Metals in atmospheric aerosols play potentially an important role in human health and ocean primary productivity. However, the lack of knowledge about solubility and speciation of metal ions in the particles or after solubilisation in aqueous media (sea or surface waters, cloud or rain droplets, biological fluids) limits our understanding of the underlying physico-chemical processes. In this work, a wide range of metals, their soluble fractions, and inorganic/organic compounds contained in urban particulate matter (PM) from Padua (Italy) were determined. Metal solubility tests have been performed by dissolving the PM in water and in solutions simulating rain droplet composition. The water-soluble fractions of the metal ions and of the organic compounds having ligand properties have been subjected to a multivariate statistical procedure, in order to elucidate associations among the aqueous concentrations of these PM components in simulated rain droplets. In parallel, a multi-dimensional speciation calculation has been performed to identify the stoichiometry and the amount of metal-ligand complexes theoretically expected in aqueous solutions. Both approaches showed that the solubility and the aqueous speciation of metal ions were differently affected by the presence of inorganic and organic ligands in the PM. The solubility of Al, Cr, and Fe was strongly correlated to the concentrations of oxalic acid, as their oxalate complexes represented the expected dominant species in aqueous solutions. Oxalates of Al represented ~98% of soluble Al, while oxalates of Cu represented 34-75% of the soluble Cu, and oxalates of Fe represented 76% of soluble Fe. The oxidation state of Fe can strongly impact the speciation picture. If Fe is present as Fe(II) rather than Fe(III), the amount of Cr and Cu complexed with diacids can increase from 75% to 94%, and from 32% to 53%, respectively. For other metals, the solubility depended on the formation of soluble aquo-complexes, hence with a scarce effect of the organic ligands. An iron-oxalate complex was also directly detected in aerosol sample extracts.

  Combined high degree of carboxylation and electronic conduction in graphene acid set new limits for metal free catalysis in alcohol oxidation

Matías Blanco, Dario Mosconi, Michal Otyepka, Miroslav Medveď, Aristides Bakandritsos, Stefano Agnoli and Gaetano Granozzi

Chem. Sci., 2019, (IF: 9.556)

DOI: 10.1039/C9SC02954K

Graphene oxide, the most prominent carbocatalyst for several oxidation reactions, has severe limitations due to the overstoichiometric amounts required to achieve practical conversions. Graphene Acid, a well-defined graphene derivative selectively and homogeneously covered by carboxylic groups but maintaining the high electronic conductivity of pristine graphene, sets new activity limits in the selective and general oxidation of a large gamut of alcohols, even working at 5 wt% loading for at least 10 reaction cycles without any influence from  metal impurities. According to experimental data and first principle calculations, the selective and dense functionalization with carboxyl groups, combined with excellent electron transfer properties, accounts for the unprecedented catalytic activity of this graphene derivative. Moreover, the controlled structure of graphene acid allows shedding light upon the critical steps of the reaction and regulating precisely its selectivity toward different oxidation products.

  Exploring the phase-selective, green hydrothermal synthesis of upconverting doped sodium yttrium fluoride: effects of temperature, time and precursors

Nora Jannsen, Stefano Diodati, Nicola Dengo, Francesca Tajoli, Nicola Vicentini, Giacomo Lucchini, Adolfo Speghini, Denis Badocco, Paolo Pastore, Silvia Gross

Chem. Eur. J., Just Accepted Manuscript

DOI: 10.1002/chem.201903261

I. F.: 5.160

Upconverting materials are materials capable, when excited with radiation of a suitable wavelength, of emitting light at a lower wavelength (and therefore at higher energy) than the incident radiation. This characteristic makes them effective candidates for many technological applications, for instance in photonics, optical codification and sensing applications. Among upconverting materials, NaYF4 is one of the most efficient.
In this paper, an easy, green low-temperature route was optimized for the synthesis of upconverting doped NaYF4, allowing to tune, through variations of the experimental parameters, the prevalence and co-occurrence of the two main crystalline phases displayed by the species. Instead of using carcinogen or toxic solvents, the synthesis was performed in water at relatively low temperatures via the hydrothermal approach. The effect of temperature, treatment time and precursors on the crystal structure, morphology and composition of the resulting materials was furthermore investigated via a plethora of techniques (XRD, XPS, SEM, ICP-MS) and the optimal parameters to yield hexagonal sodium yttrium fluoride were determined.

  The mechanism of concentric HfO2/Co3O4/TiO2 nanotubes investigated by intensity modulated photocurrent spectroscopy (IMPS) and electrochemical impedance spectroscopy (EIS) for photoelectrochemical activity

Xiaochen Huai, Leonardo Girardi, Ran Lu, Shang Gao, Yu Zhao, Yunhan Ling, Gian Andrea Rizzi, Gaetano Granozzi, Zhengjun Zhang

NanoEnergy 2019, (IF: 15.548)

https://doi.org/10.1016/j.nanoen.2019.104020

In this work, we describe the preparation and photoelectrochemical (PEC) properties of concentric HfO2/Co3O4/TiO2 nanotubes (NTs). From a methodological point of view, we demonstrated, for the first time, that intensity modulated photocurrent spectroscopy (IMPS) and electrochemical impedance spectroscopy (EIS) efficiently complement and can be highly valuable to optimize PEC water-splitting devices. We verified that the CSC and CH values (the capacitance of space charge region and Helmholtz layer, derived from a phenomenological equivalent circuits model, ECM) are of similar magnitude and that the CH can be affected by applied bias, the nature of the catalytic material and the passivation layer. The calculated CH/(CH+CSC) are in agreement with the high frequency intercept (HFI) derived by the IMPS experimental data.
The charge transfer efficiencies (ղtr) were obtained from the IMPS and ECM data: the ղtr values for HfO2/Co3O4/TiO2-NTs increased to 80.4% (0 V vs Ag/AgCl) compared to those in Co3O4/TiO2-NTs (56.9%,0 V vs Ag/AgCl) and in TiO2-NT (25.6%, 0 V vs Ag/AgCl).
We proved that the excellent PEC properties of HfO2/Co3O4/TiO2-NTs are due to i) the band alignment between small bad gap of Co3O4 and TiO2 semiconductors, ii) increased hole transfer efficiency ղtr caused by excellent catalytic properties of Co3O4 and iii) the passivation effect on surface defects of a HfO2 layer with appropriate thickness.