Surface area characterization of obliquely deposited metal oxide nanostructured thin films

Innovative Energy Storage Smart Windows Relying on Mild Aqueous Zn/MnO2 Battery Chemistry

Rechargeable mild aqueous Zn/MnO2 batteries are currently attracting great interest thanks to their appealing performance/cost ratio. Their operating principle relies on two complementary reversible electrodeposition reactions at the anode and cathode. Transposing this operating principle to transparent conductive windows remains an unexplored facet of this battery chemistry, which is proposed here to address with the development of an innovative bifunctional smart window, combining electrochromic and charge storage properties. The proof-of-concept of such bifunctional Zn/MnO2 smart window is provided using a mild buffered aqueous electrolyte and different architectures. To maximize the device's performance, transparent nanostructured ITO cathodes are used to reversibly electrodeposit a high load of MnO2 (up to 555 mA h m-2 with a CE of 99.5% over 200 cycles, allowing to retrieve an energy density as high as 860 mA h m-2 when coupled with a zinc metal frame), while flat transparent FTO anodes are used to reversibly electrodeposit an homogenous coating of zinc metal (up to ≈280 mA h m-2 with a CE > 95% over 50 cycles). The implementation of these two reversible electrodeposition processes in a single smart window has been successfully achieved, leading for the first time to a dual-tinting energy storage smart window with an optimized face-to-face architecture.

The Role of Al3+ -Based Aqueous Electrolytes in the Charge Storage Mechanism of MnOx Cathodes

Rechargeable aqueous aluminium batteries are the subject of growing interest, however, the charge storage mechanisms at manganese oxide-based cathodes remain poorly understood. In essense, every study proposes a different mechanism. Here, an in situ spectroelectrochemical methodology is used to unambiguously demonstrate that reversible proton-coupled MnO₂ -to-Mn²⁺ conversion is the main charge storage mechanism occurring at MnO₂ cathodes for a range of slightly acidic Al³⁺ -based aqueous electrolytes, with the Al³⁺ hexaaquo complex playing the key role of proton donor. In Zn/MnO₂ assemblies, this mechanism is associated with high gravimetric capacities and discharge potentials, up to 560 mAh g^-1 and 1.65 V respectively, attractive efficiencies (CE > 99.5% and EE > 82%) and excellent cyclability (almost 100% capacity retention over 1 400 cycles at 2 A g^-1 ). Finally, a critical analysis of the data previously published on MnO_x cathodes in Al³⁺ -based aqueous electrolytes is conducted to conclude on a universal charge storage mechanism, i.e., the reversible electrodissolution/electrodeposition of MnO₂ .

Porosimetry for Thin Films of Metal-Organic Frameworks: A Comparison of Positron Annihilation Lifetime Spectroscopy and Adsorption-Based Methods

Thin films of crystalline and porous metal-organic frameworks (MOFs) have great potential in membranes, sensors, and microelectronic chips. While the morphology and crystallinity of MOF films can be evaluated using widely available techniques, characterizing their pore size, pore volume, and specific surface area is challenging due to the low amount of material and substrate effects. Positron annihilation lifetime spectroscopy (PALS) is introduced as a powerful method to obtain pore size information and depth profiling in MOF films. The complementarity of this approach to established physisorption-based methods such as quartz crystal microbalance (QCM) gravimetry, ellipsometric porosimetry (EP), and Kr physisorption (KrP) is illustrated. This comprehensive discussion on MOF thin film porosimetry is supported by experimental data for thin films of ZIF-8.

Qualitative Approaches Towards Useful Photocatalytic Materials

The long-standing crusade searching for efficient photocatalytic materials has resulted in a vast landscape of promising photocatalysts, as reflected by the number of reviews reported in the last decade. Virtually all of these reviews have focused on quantitative approaches aiming at developing an understanding of the underlying mechanisms behind photocatalytic behavior and the parameters that influence structure–function correlation. Less attention has been paid, however, to qualitative measures around the development and assessment of photocatalysts. These measures will contribute toward narrowing the range of potential photocatalytic materials for widespread applications. The current report provides a critical perspective over some of the main factors affecting the assessment of photocatalytic materials as a code of good practice. A case of study is also provided, where this qualitative analysis is applied to one of the most prolific materials of the last-decade, disorder-engineered, black titanium dioxide (TiO₂).

Peroxidase-Like Behavior of Ni Thin Films Deposited by Glancing Angle Deposition for Enzyme-Free Uric Acid Sensing

We present a nanozyme-based biosensor fabricated from nanostructured Ni films deposited onto a silicon wafer by glancing angle deposition (GLAD) for enzyme-free colorimetric monitoring of uric acid (UA), a biomarker for gout, high blood pressure, heart disease, and kidney disease. The helically structured Ni GLAD nanozymes exhibit excellent peroxidase-like activity to accelerate the oxidation reaction of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to a blue product, oxidized TMB (oxTMB), mediated by H₂O₂. In the presence of UA, oxTMB is reduced, decreasing the optical absorbance by an amount determined by the concentration of UA in the solution. The nanozyme not only mimics peroxidase but also possesses the notable qualities of reusability, simple operation, and reliability, making it environment-friendly and suitable for on-demand analysis. We optimized essential working parameters (pH, TMB concentration, and H₂O₂ concentration) to maximize the initial color change of the TMB solution. The catalytic activity of this nanozyme was compared with conventional nanofilms using the Michaelis-Menten theory. Based on this, enzyme-free biosensors were developed for colorimetric detection of UA, providing a wide detection range and a limit of detection (3.3 μM) suitable for measurements of UA concentration in sweat. Furthermore, interference from glucose and urea was studied so as to explore the potential of the biosensor for use in the clinical diagnosis of UA biomarkers.

Nanostructured nickel oxide electrodes for non-enzymatic electrochemical glucose sensing

Nanostructured nickel (Ni) and nickel oxide (NiO) electrodes were fabricated on Ni foils using the glancing angle deposition (GLAD) technique. Cyclic voltammetry and amperometry showed the electrodes enable non-enzymatic electrochemical determination of glucose in strongly alkaline media. Under optimized conditions of NaOH concentration and working potential (~ 0.50 V vs. Ag/AgCl), the GLAD electrodes performed far better than bare Ni foil electrodes, with the GLAD NiO electrode showing an outstanding sensitivity (4400 μA mM^-1 cm^-2), superior detection limit (7 nM), and wide dynamic range (0.5 μM-9 mM), with desirable selectivity and reproducibility. Based on their performance at a low concentration, the GLAD NiO electrodes were also used to quantify glucose in artificial urine and sweat samples which have significantly lower glucose levels than blood. The GLAD NiO electrodes showed negligible response to the common interferents in glucose measurement (uric acid, dopamine, serotonin, and ascorbic acid), and they were not poisoned by high amounts of sodium chloride. Graphical abstract The figures depict (A) SEM image of vertical post-GLAD NiO electrodes used for non-enzymatic electrochemical glucose monitoring, and (B) calibration plots of the three different electrodes.

Structural Characterization of Mesoporous Thin Film Architectures: A Tutorial Overview

Mesoporous thin film architectures are an important class of materials that exhibit unique properties, which include high surface area, versatile surface functionalization, and bicontinuous percolation paths through a broad library of pore arrangements on the 10 nm length scale. Although porosimetry of bulk materials via sorption techniques is common practice, the characterization of thin mesoporous films with small sample volumes remains a challenge. A range of techniques are geared toward providing information over pore morphology, pore size distribution, surface area and overall porosity, but none of them offers a holistic evaluation and results are at times inconsistent. In this work, we present a tutorial overview for the reliable structural characterization of mesoporous films. Three model samples with variable pore size and porosity prepared by block copolymer (BCP) coassembly serve for a rational comparison. Various techniques are assessed side-by-side, including scanning electron microscopy (SEM), atomic force microscopy (AFM), grazing incidence small-angle X-ray scattering (GISAXS), and ellipsometric porosimetry (EP). We critically discuss advantages and limitations of each technique and provide guidelines for reliable implementation.

Influence of Thickness and Sputtering Pressure on Electrical Resistivity and Elastic Wave Propagation in Oriented Columnar Tungsten Thin Films

Tungsten films were prepared by DC magnetron sputtering using glancing angle deposition with a constant deposition angle α = 80°. A first series of films was obtained at a constant pressure of 4.0 × 10^-3 mbar with the films' thickness increasing from 50 to 1000 nm. A second series was produced with a constant thickness of 400 nm, whereas the pressure was gradually changed from 2.5 × 10^-3 to 15 × 10^-3 mbar. The A15 β phase exhibiting a poor crystallinity was favored at high pressure and for the thinner films, whereas the bcc α phase prevailed at low pressure and for the thicker ones. The tilt angle of the columnar microstructure and fanning of their cross-section were tuned as a function of the pressure and film thickness. Electrical resistivity and surface elastic wave velocity exhibited the highest anisotropic behaviors for the thickest films and the lowest pressure. These asymmetric electrical and elastic properties were directly connected to the anisotropic structural characteristics of tungsten films. They became particularly significant for thicknesses higher than 450 nm and when sputtered particles were mainly ballistic (low pressures). Electronic transport properties, as well as elastic wave propagation, are discussed considering the porous architecture changes vs. film thickness and pressure.

On the unsuspected role of multivalent metal ions on the charge storage of a metal oxide electrode in mild aqueous electrolytes

Insertion mechanisms of multivalent ions in transition metal oxide cathodes are poorly understood and subject to controversy and debate, especially when performed in aqueous electrolytes. To address this issue, we have here investigated the reversible reduction of nanostructured amorphous TiO2 electrodes by spectroelectrochemistry in mild aqueous electrolytes containing either a multivalent metal salt as AlCl3 or a weak organic acid as acetic acid. Our results show that the reversible charge storage in TiO2 is thermodynamically and kinetically indistinguishable when carried out in either an Al3+- or acetic acid-based electrolyte, both leading under similar conditions of pH and concentrations to an almost identical maximal charge storage of ∼115 mA h g-1. These observations are in agreement with a mechanism where the inserting/deinserting cation is the proton and not the multivalent metal cation. Analysis of the data also demonstrates that the proton source is the Brønsted weak acid present in the aqueous electrolyte, i.e. either the acetic acid or the aquo metal ion complex generated from solvation of Al3+ (i.e. [Al(H2O)6]3+). Such a proton-coupled charge storage mechanism is also found to occur with other multivalent metal ions such as Zn2+ and Mn2+, albeit with a lower efficiency than Al3+, an effect we have attributed to the lower acidity of [Zn(H2O)6]2+ and [Mn(H2O)6]2+. These findings are of fundamental importance because they shed new light on previous studies assuming reversible Al3+-insertion into metal oxides, and, more generally, they highlight the unsuspected proton donor role played by multivalent metal cations commonly involved in rechargeable aqueous batteries.

Synthesis of nanostructured LiMn2O4 thin films by glancing angle deposition for Li-ion battery applications

The development of electric vehicles and portable electronic devices demands lighter and thinner batteries with improved specific charge and rate capabilities. In this work, thin films of LiMn₂O₄ were fabricated by rf magnetron sputtering. Glancing angle deposition is introduced as a promising approach for fabrication of porous cathode thin films with 2.6 times the capacity in comparison with conventionally sputtered films of the same thickness. Surface morphology and crystallinity of the films are studied along with their electrochemical performance in an aqueous electrolyte. The glancing angle deposited films can reach a rate capability of up to 4 mA cm^-2 with minimal energy loss, and a life cycle longer than 100 charge/discharge cycles.

Linearly polarized Q-switched ceramic laser made with anisotropic nanostructured thin films

A polarizing laser mirror was made of an alternating sequence of low and high refractive index layers of titanium oxide using glancing angle deposition (GLAD). Large refractive index contrast and large birefringence, reaching 0.5 and 0.1, respectively, could be obtained from one single raw material by changing the deposition conditions. The laser mirror could withstand a train of 2.7 ns, single-mode pulses at 680 Hz, λ=1030  nm, and peak power density of 670  MW/cm² when used as an output coupler of a passively Q-switched (Yb_0.1Y_0.9)₃Al₅O₁₂ ceramic laser. The polarization extinction ratio was found to be better than 30 dB both in continuous-wave and pulsed regimes. These results indicate that polarizing laser mirrors made from nanostructured thin films with GLAD, in addition to being simple to fabricate, can withstand high pulse energy density.

Enzymatically active biomimetic micropropellers for the penetration of mucin gels

In the body, mucus provides an important defense mechanism by limiting the penetration of pathogens. It is therefore also a major obstacle for the efficient delivery of particle-based drug carriers. The acidic stomach lining in particular is difficult to overcome because mucin glycoproteins form viscoelastic gels under acidic conditions. The bacterium Helicobacter pylori has developed a strategy to overcome the mucus barrier by producing the enzyme urease, which locally raises the pH and consequently liquefies the mucus. This allows the bacteria to swim through mucus and to reach the epithelial surface. We present an artificial system of reactive magnetic micropropellers that mimic this strategy to move through gastric mucin gels by making use of surface-immobilized urease. The results demonstrate the validity of this biomimetic approach to penetrate biological gels, and show that externally propelled microstructures can actively and reversibly manipulate the physical state of their surroundings, suggesting that such particles could potentially penetrate native mucus.

Single-step fabrication process of 1-D photonic crystals coupled to nanocolumnar TiO2 layers to improve DSC efficiency

The present work proposes the use of a TiO₂ electrode coupled to a one-dimensional photonic crystal (1DPC), all formed by the sequential deposition of nanocolumnar thin films by physical vapor oblique angle deposition (PV-OAD), to enhance the optical and electrical performance of DSCs while transparency is preserved. We demonstrate that this approach allows building an architecture combining a non-dispersive 3 µm of TiO₂ electrode and 1 µm TiO₂-SiO₂ 1DPC, both columnar, in a single-step process. The incorporation of the photonic structure is responsible for a rise of 30% in photovoltaic efficiency, as compared with a transparent cell with a single TiO₂ electrode. Detailed analysis of the spectral dependence of the photocurrent demonstrates that the 1DPC improves light harvesting efficiency by both back reflection and optical cavity modes confinement within the TiO₂ films, thus increasing the overall performance of the cell.

Oxygen Evolution Catalyzed by Nickel-Iron Oxide Nanocrystals with a Nonequilibrium Phase

Mixed nickel-iron oxides are of great interest as electrocatalysts for the oxygen evolution reaction (OER), the kinetically challenging half-reaction required for the generation of hydrogen gas from water via electrolysis. Previously, we had reported the synthesis of single crystal, soluble nickel-iron oxide nanoparticles over a wide range of nickel:iron compositions, with a metastable cubic rock salt phase ([Ni,Fe]O) that can be isolated despite the low solubility of iron in cubic nickel oxide at ambient temperatures. Here, activity for OER was examined, catalyzed by these [Ni,Fe]O nanoparticles integrated with indium tin oxide (ITO) electrodes. Because the as-prepared [Ni,Fe]O nanoparticles are oleate-capped, the surface ligands needed to be removed to induce adherence to the ITO substrate, and to enable charge transfer and contact with water to enable OER catalysis. Two different approaches were taken to reduce or eliminate the coverage of oleate ligands in these films: UV irradiation (254 nm) and air plasma. UV irradiation proved to lead to better results in terms of stable and OER-active films at pH 13. Kinetic analysis revealed that the Tafel slopes of these nanoparticle [Ni,Fe]O OER electrodes were limited by the electrochemical surface area and were found to be within the range of 30 to 50 mV/decade. Across the four compositions of Ni:Fe studied, from 24:76 to 88:12, the observed overpotential at 10 mA/cm2 for the OER in basic conditions decreased from 0.47 to 0.30 V as the proportion of nickel increased from 24% to 88%.

Tuning the reactivity of nanostructured indium tin oxide electrodes toward chemisorption

This contribution highlights correlation between the surface concentration of a chemisorbed organophosphorous probe (flavin mononucleotide) and the relative hydroxyl surface coverage of nanostructured ITO electrodes, which can be tuned during post-deposition reductive annealing.

Enhanced photoelectrochemical properties of TiO2 nanorod arrays decorated with CdS nanoparticles

TiO₂ nanorod arrays (TiO₂ NRAs) sensitized with CdS nanoparticles were fabricated via successive ion layer adsorption and reaction (SILAR), and TiO₂ NRAs were obtained by oxidizing Ti NRAs obtained through oblique angle deposition. The TiO₂ NRAs decorated with CdS nanoparticles exhibited excellent photoelectrochemical and photocatalytic properties under visible light, and the one decorated with 20 SILAR cycles CdS nanoparticles shows the best performance. This can be attributed to the enhanced separation of electrons and holes by forming heterojunctions of CdS nanoparticles and TiO₂ NRAs. This provides a promising way to fabricate the material for solar energy conversion and wastewater degradation.

Wafer-scale, three-dimensional helical porous thin films deposited at a glancing angle

Minimization of helices opens a door to impose novel functions derived from the dimensional shrinkage of optical, mechanical and electronic devices. Glancing angle deposition (GLAD) enables one to deposit three-dimensional helical porous thin films (HPTFs) composed of separated spiral micro/nano-columns. GLAD integrates a series of advantageous features, including one-step deposition, wafer-scale production with mono-handedness of spirals, flexible engineering of spiral materials and dimensions, and the adaption to various kinds of substrates. Herein, we briefly review the fabrication of HPTFs by GLAD, specific growth mechanisms, physical properties in structures, mechanics and chiral optics, and the emerging applications in green energy. A prospective outlook is presented to illuminate some promising developments in enantioselection, bio-dynamic analyses, wirelessly-controlled drug delivery and mass production.

Spectroelectrochemistry of FeIII- and CoIII-mimochrome VI artificial enzymes immobilized on mesoporous ITO electrodes

UV-visible absorption spectroelectrochemistry elucidated the different redox behaviours of Fe^III- and Co^III-mimochrome VI artificial enzymes, adsorbed on mesoporous conductive films of ITO.

Ultrathin-layer chromatography nanostructures modified by atomic layer deposition

Stationary phase morphology and surface chemistry dictate the properties of ultrathin-layer chromatography (UTLC) media and interactions with analytes in sample mixtures. In this paper, we combined two powerful thin film deposition techniques to create composite chromatography nanomaterials. Glancing angle deposition (GLAD) produces high surface area columnar microstructures with aligned macropores well-suited for UTLC. Atomic layer deposition (ALD) enables precise fabrication of conformal, nanometer-scale coatings that can tune surfaces of these UTLC films. We coated ∼5μm thick GLAD SiO2 UTLC media with <10nm thick ALD metal oxides (Al2O3, ZrO2, and ZnO) to decouple surface chemistry from the underlying GLAD scaffold microstructure. The effects of ALD coatings on GLAD UTLC media were investigated using transmission electron microscopy (TEM), gas adsorption porosimetry, and lipophilic dye separations. The results collectively show that the most significant changes occur over the first few nanometers of ALD coating. They further demonstrate independent control of film microstructure and surface characteristics. ALD coatings can enhance complex GLAD microstructures to engineer new composite nanomaterials potentially useful in analytical chromatography.

Dye-Sensitized Nanostructured Crystalline Mesoporous Tin-doped Indium Oxide Films with Tunable Thickness for Photoelectrochemical Applications

A simple route towards nanostructured mesoporous Indium-Tin Oxide (templated nano-ITO) electrodes exhibiting both high conductivities and optimized bicontinuous pore-solid network is reported. The ITO films are first produced as an X-ray-amorphous, high surface area material, by adapting recently established template-directed sol-gel methods using Sn(IV) and In(III) salts. Carefully controlled temperature/atmosphere treatments convert the as-synthesized ITO films into nano-crystalline coatings with the cubic bixbyite structure. Specially, a multi-layered synthesis was successfully undertaken for tuning the film thickness. In order to evaluate the performances of templated nano-ITO as an electrode substrate for photoelectrochemical applications, photoelectrodes were prepared by covalent grafting of a redox-active dye, the complex [Ru(bpy)₂(4,4'-(CH₂PO₃H₂)₂-bpy)]Cl₂1 (bpy=bipyridine). Surface coverage was shown to increase with the film thickness, from 0.7 × 10^-9 mol.cm^-2 (one layer, 45 nm) to 3.5 × 10^-9 mol.cm^-2 (ten layers, 470 nm), the latter value being ~ 100 times larger than that for commercially available planar ITO. In the presence of an electron mediator, photocurrents up to 50 μA.cm^-2 have been measured under visible light irradiation, demonstrating the potential of this new templated nano-ITO preparation for the construction of efficient photoelectrochemical devices.

Tailored nanoporous coatings fabricated on conformable polymer substrates

Nanoporous coatings have become the subject of intense investigation, in part because they have been shown to have unique and tailorable physical properties that can depart greatly from their dense or macroscopic counterparts. Nanoporous coatings are frequently fabricated utilizing oblique-angle or glancing-angle physical vapor-phase deposition techniques. However, a significant limitation for such coatings exists; they are almost always deposited on smooth and rigid planar substrates, such as silicon and glass. This limitation greatly constrains the applicability, tailorability, functionality and even the economic viability, of such nanoporous coatings. Here, we report our findings on nanoporous/polymer composite systems (NPCS) fabricated by utilizing oblique-angle electron-beam methodology. These unique composite systems exhibit several favorable characteristics, namely, (i) fine-tuned control over coating nanoporosity and thickness, (ii) excellent adhesion between the nanoporous coating and polymer substrate, (iii) the ability to withstand significant and repeated bending, and (iv) the ability to be molded conformably on two and three-dimensional surfaces while closely retaining the composite system's designed nanoporous film structure and, hence, properties.

Spectroelectrochemical characterization of small hemoproteins adsorbed within nanostructured mesoporous ITO electrodes

3D nanostructured transparent indium tin oxide (ITO) electrodes prepared by glancing angle deposition (GLAD) were used for the spectroelectrochemical characterization of cytochrome c (Cyt c) and neuroglobin (Nb). These small hemoproteins, involved as electron-transfer partners in the prevention of apoptosis, are oppositely charged at physiological pH and can each be adsorbed within the ITO network under different pH conditions. The resulting modified electrodes were investigated by UV-visible absorption spectroscopy coupled with cyclic voltammetry. By using nondenaturating adsorption conditions, we demonstrate that both proteins are capable of direct electron transfer to the conductive ITO surface, sharing apparent standard potentials similar to those reported in solution. Preservation of the 3D protein structure upon adsorption was confirmed by resonance Raman (rR) spectroscopy. Analysis of the derivative cyclic voltabsorptograms (DCVA) monitored either in the Soret or the Q bands at scan rates up to 1 V s(-1) allowed us to investigate direct interfacial electron transfer kinetics. From the DCVA shape and scan rate dependences, we conclude that the interaction of Cyt c with the ITO surface is more specific than Nb, suggesting an oriented adsorption of Cyt c and a random adsorption of Nb on the ITO surface. At the same time, Cyt c appears more sensitive to the experimental adsorption conditions, and complete denaturation of Cyt c may occur as evidenced from cross-correlation of rR spectroscopy and spectroelectrochemistry.

Unraveling the mechanism of catalytic reduction of O2 by microperoxidase-11 adsorbed within a transparent 3D-nanoporous ITO film

Nanoporous films of indium tin oxide (ITO), with thicknesses ranging from 250 nm to 2 μm, were prepared by Glancing Angle Deposition (GLAD) and used as highly sensitive transparent 3D-electrodes for quantitatively interrogating, by time-resolved spectroelectrochemistry, the reactivity of microperoxidase-11 (MP-11) adsorbed within such films. The capacitive current densities of these 3D-electrodes as well as the amount of adsorbed MP-11 were shown to be linearly correlated to the GLAD ITO film thickness, indicating a homogeneous distribution of MP-11 across the film as well as homogeneous film porosity. Under saturating adsorption conditions, MP-11 film concentration as high as 60 mM was reached. This is equivalent to a stack of 110 monolayers of MP-11 per micrometer film thickness. This high MP-11 film loading combined with the excellent ITO film conductivity has allowed the simultaneous characterization of the heterogeneous one-electron transfer dynamics of the MP-11 Fe(III)/Fe(II) redox couple by cyclic voltammetry and cyclic voltabsorptometry, up to a scan rate of few volts per second with a satisfactory single-scan signal-to-noise ratio. The potency of the method to unravel complex redox coupled chemical reactions was also demonstrated with the catalytic reduction of oxygen by MP-11. In the presence of O(2), cross-correlation of electrochemical and spectroscopic data has allowed us to determine the key kinetics and thermodynamics parameters of the redox catalysis that otherwise could not be easily extracted using conventional protein film voltammetry. On the basis of numerical simulations of cyclic voltammograms and voltabsorptograms and within the framework of different plausible catalytic reaction schemes including appropriate approximations, it was shown possible to discriminate between different possible catalytic pathways and to identify the relevant catalytic cycle. In addition, from the best fits of simulations to the experimental voltammograms and voltabsorptograms, the partition coefficient of O(2) for the ITO film as well as the values of two kinetic rate constants could be extracted. It was finally concluded that the catalytic reduction of O(2) by MP-11 adsorbed within nanoporous ITO films occurs via a 2-electron mechanism with the formation of an intermediate Fe(III)-OOH adduct characterized by a decay rate of 11 s(-1). The spectroelectroanalytical strategy presented here opens new opportunities for characterizing complex redox-coupled chemical reactions not only with redox proteins, but also with redox biomimetic systems and catalysts. It might also be of great interest for the development and optimization of new spectroelectrochemical sensors and biosensors, or eventually new photoelectrocatalytic systems or biofuel cells.

Reactive ballistic deposition of nanostructured model materials for electrochemical energy conversion and storage

Porous, high surface area materials have critical roles in applications including catalysis, photochemistry, and energy storage. In these fields, researchers have demonstrated that the nanometer-scale structure modifies mechanical, optical, and electrical properties of the material, greatly influencing its behavior and performance. Such complex chemical systems can involve several distinct processes occurring in series or parallel. Understanding the influence of size and structure on the properties of these materials requires techniques for producing clean, simple model systems. In the fields of photoelectrochemistry and lithium storage, for example, researchers need to evaluate the effects of changing the electrode structure of a single material or producing electrodes of many different candidate materials while maintaining a distinctly favorable morphology. In this Account, we introduce our studies of the formation and characterization of high surface area, porous thin films synthesized by a process called reactive ballistic deposition (RBD). RBD is a simple method that provides control of the morphology, porosity, and surface area of thin films by manipulating the angle at which a metal-vapor flux impinges on the substrate during deposition. This approach is largely independent of the identity of the deposited material and relies upon limited surface diffusion during synthesis, which enables the formation of kinetically trapped structures. Here, we review our results for the deposition of films from a number of semiconductive materials that are important for applications such as photoelectrochemical water oxidation and lithium ion storage. The use of RBD has enabled us to systematically control individual aspects of both the structure and composition of thin film electrodes in order to probe the effects of each on the performance of the material. We have evaluated the performance of several materials for potential use in these applications and have identified processes that limit their performance. Use of model systems, such as these, for fundamental studies or materials screening processes likely will prove useful in developing new high-performance electrodes.

Indium tin oxide nanowhisker morphology control by vapour-liquid-solid glancing angle deposition

A new growth technique for indium tin oxide nanowhiskers with increased control over feature size and spacing is reported. The technique is based on a unique combination of self-catalysed vapour-liquid-solid (VLS) growth and glancing angle deposition (GLAD). This VLS-GLAD technique provides enhanced control over nanowhisker morphology as the effect of typical VLS growth parameters (e.g. flux rate, temperature) is amplified at large deposition angles characteristic of GLAD. Spatial modulation of the collimated growth flux controls trunk width, number and orientation of branches, and overall nanowhisker density. Here we report the influence of growth conditions (including deposition angle, flux rate, nominal pitch and substrate temperature) on nanowhisker morphology, with specific focus on the effect of large deposition angles. Sheet resistance and transmission of the films were measured to characterize their performance as transparent conductive oxides. Hybrid nanostructured films grown in this study include high surface area nanowhiskers protruding from a conductive film, ideal for transparent conductive electrode applications.

Glancing angle deposited ITO films for efficiency enhancement of a-Si:H/μc-Si:H tandem thin film solar cells

Indium tin oxide (ITO) thin films with relatively high transparency and low absorption are prepared by glancing angle deposition (GLAD) method and their effect on the device performance of a-Si:H/μc-Si:H tandem thin film solar cells is theoretically investigated by applying the experimentally measured physical data of the fabricated films to the simulation parameters. The GLAD of ITO produces inclined porous columnar nanostructures due to the atomic shadowing effect. With increasing the incident flux angle, the columns are increasingly inclined, thus resulting in the improved transmission property as well as the decrease of the refractive index and extinction coefficient because of enhanced porosity within the film. Furthermore, the antireflection characteristics are improved over a wide wavelength range of 300-1100 nm. For a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the 0° ITO/80° ITO bi-layer structure, the conversion efficiency (η) of 13.6% is obtained from simulation under AM1.5g illumination, indicating an efficiency improvement compared to the device with the 0° ITO/0° ITO bi-layer structure (i.e. η = 12.58%).

Microchannels filled with diverse micro- and nanostructures fabricated by glancing angle deposition

The integration of porous structures into microchannels is known to enable unique and useful separations both in electrophoresis and chromatography. Etched pillars and other nanostructures have received considerable interest in recent years as a platform for creating microchannels with pores tailored to specific applications. We present a versatile method for integration of three-dimensionally sculptured nano- and micro-structures into PDMS microchannels. Glancing angle deposition was used to fabricate nanostructures that were subsequently embedded in PDMS microchannels using a sacrificial resist process. With this technique, an assortment of structures made from a wide selection of materials can be integrated in PDMS microchannels; some examples of this versatility, including chiral and chevron nanostructures, are demonstrated. We also present a working device made using this process, separating 6/10/20 kbp and 10/48 kbp DNA mixtures in a DNA fractionator containing GLAD-deposited SiO(2) vertical posts as the separating medium. The separation mechanism was verified to resemble that found in prior fractionation devices, using total internal reflection fluorescence microscopy. GLAD fabrication enables insertion of three-dimensional structures into microchannels that cannot be fabricated with any existing techniques, and this versatility in structural design could facilitate new developments in on-chip separations.

Indium tin oxide nanopillar electrodes in polymer/fullerene solar cells

Using high surface area nanostructured electrodes in organic photovoltaic (OPV) devices is a route to enhanced power conversion efficiency. In this paper, indium tin oxide (ITO) and hybrid ITO/SiO(2) nanopillars are employed as three-dimensional high surface area transparent electrodes in OPVs. The nanopillar arrays are fabricated via glancing angle deposition (GLAD) and electrochemically modified with nanofibrous PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate)). The structures are found to have increased surface area as characterized by porosimetry. When applied as anodes in polymer/fullerene OPVs (architecture: commercial ITO/GLAD ITO/PEDOT:PSS/P3HT:PCBM/Al, where P3HT is 2,5-diyl-poly(3-hexylthiophene) and PCBM is [6,6]-phenyl-C(61)-butyric acid methyl ester), the air-processed solar cells incorporating high surface area, PEDOT:PSS-modified ITO nanoelectrode arrays operate with improved performance relative to devices processed identically on unstructured, commercial ITO substrates. The resulting power conversion efficiency is 2.2% which is a third greater than for devices prepared on commercial ITO. To further refine the structure, insulating SiO(2) caps are added above the GLAD ITO nanopillars to produce a hybrid ITO/SiO(2) nanoelectrode. OPV devices based on this system show reduced electrical shorting and series resistance, and as a consequence, a further improved power conversion efficiency of 2.5% is recorded.

Time-resolved UV-visible spectroelectrochemistry using transparent 3D-mesoporous nanocrystalline ITO electrodes

Efficient and rapid adsorption of microperoxidase 11 within a highly porous ITO thin film (200 nm) prepared by glancing angle deposition was achieved. Adsorbed redox molecules were reversibly and rapidly reduced throughout the 3D-conductive matrix in ca. 50 ms, allowing the heterogeneous electron transfer rate to be determined by derivative cyclic voltabsorptometry.

Epitaxial growth of nanostructured gold films on germanium via galvanic displacement

This work focuses on the synthesis and characterization of gold films grown via galvanic displacement on Ge(111) substrates. The synthetic approach uses galvanic displacement, a type of electroless deposition that takes place in an efficient manner under aqueous, room temperature conditions. Investigations involving X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques were performed to study the crystallinity and orientation of the resulting gold-on-germanium films. A profound effect of HF(aq) concentration was noted, and although the SEM images did not show significant differences in the resulting gold films, a host of X-ray diffraction studies demonstrated that higher concentrations of HF(aq) led to epitaxial gold-on-germanium, whereas in the absence of HF(aq), lower degrees of order (fiber texture) resulted. Cross-sectional nanobeam diffraction analyses of the Au-Ge interface confirmed the epitaxial nature of the gold-on-germanium film. This epitaxial behavior can be attributed to the simultaneous etching of the germanium oxides, formed during the galvanic displacement process, in the presence of HF. High-resolution TEM analyses showed the coincident site lattice (CSL) interface of gold-on-germanium, which results in a small 3.8% lattice mismatch due to the coincidence of four gold lattices with three of germanium.

Taking a little off the top: nanorod array morphology and growth studied by focused ion beam tomography

The high surface area, large aspect ratio, and porous nature of nanorod arrays make them excellent foundation materials for many devices. Of the many synthesis techniques for forming nanorods, glancing angle deposition (GLAD) offers one of the more straightforward and flexible methods for ensuring control of alignment, porosity, and architecture of the nanorods. Here we demonstrate the first use of a dual-beam (focused ion beam (FIB) combined with scanning electron microscopy (SEM)) instrument to section and image the internal morphology of a nanorod array fabricated using the GLAD technique. We have used the FIB-SEM to reconstruct the 3D composition of TiO(2) nanorods, allowing us to visualize for the first time the core structures of many potential devices. We have also been able to probe the relationship between critical parameters such as diameter (w(act)), internanorod spacing (ν(act)), center-to-center spacing (c(act)), and nanorod population density (d(act)) and the depth of the nanocolumn (t) for a single homogeneous structure. A continuous data set was obtained from a single 5-μm-thick GLAD film, avoiding the artifacts arising from the analysis of the top surfaces of multiple samples of varying thicknesses. An analysis of the acquired sectioned data has allowed us to determine that the critical nanocolumn parameters follow a power-law scaling trend with w(act) = 9.4t(0.35) nm, ν(act) = 15.2t(0.25) nm, c(act) = 24.8t(0.31) nm, and d(act) = 3402t(-0.65) columns μm(-2). Using the FIB/SEM images acquired for the TiO(2) nanorods, we have also investigated the evolution of individual nanocolumns and have observed that bifurcation and branching play a significant role in the extinction or survival of these nanorods. These findings will allow for the optimization of nanorod properties for device applications. Also, the FIB sectioning and reconstruction process developed here will permit for the investigation of nanorod arrays formed from a range of synthesis techniques and materials.

Engineered anisotropic microstructures for ultrathin-layer chromatography

The strong dependence of separation behavior on ultrathin-layer chromatography (UTLC) stationary phase microstructure motivates continued UTLC plate design optimization efforts. We fabricated 4.6-5.3 mum thick normal phase silica UTLC stationary phases with several types of in-plane macropore anisotropies using the glancing angle deposition (GLAD) approach to engineering nanostructured thin films. The separation behaviors of two new media, isotropic vertical posts and anisotropic bladelike films, were compared to that of anisotropic chevron media. Channel-like structures within the anisotropic media introduced preferential mobile phase flow directions that could be exploited to give separation tracks diagonal to the development direction. Extraction of chromatograms from these angled tracks required the development of a new analytical approach that involved a commercial flatbed film scanner and custom numerical image analysis software. GLAD stationary phase performance was quantified using the Dimethyl Yellow dye separated from a lipophilic dye mixture over migration distances less than approximately 10 mm. The limits of detection were 10 +/- 4 ng for the vertical posts and 11 +/- 3 ng for the bladelike media. We obtained theoretical plate heights that varied with film microstructure between 12 and 28 mum. Unoptimized separation performance was comparable to that of other planar chromatography media. Macropore anisotropies engineered by GLAD may expand the capabilities of future UTLC stationary phases.

Matrix-free laser desorption/ionization mass spectrometry using silicon glancing angle deposition (GLAD) films

Glancing angle deposition (GLAD) was used to fabricate nanostructured silicon (Si) thin films with highly controlled morphology for use in laser desorption/ionization mass spectrometry (DIOS-MS). Peptides, drugs and metabolites in the mass range of 150-2500 Da were readily analyzed. The best performance was obtained with 500 nm thick films deposited at a deposition angle of 85 degrees . Low background mass spectra and attomole detection limits were observed with DIOS-MS for various peptides. Films used after three months of dry storage in ambient conditions produced mass spectra with negligible low-mass noise following a 15 min UV-ozone treatment. The performance of the Si GLAD films was as good as or better than that reported for electrochemically etched porous silicon and related materials, and was superior to matrix-assisted laser desorption/ionization (MALDI)-MS for analysis of mixtures of small molecules between 150-2500 Da in terms of background chemical noise, detection limits and spot-to-spot reproducibility. The spot-to-spot reproducibility of signal intensities (100 shots/spectrum) from 21 different Si GLAD film targets was +/-13% relative standard deviation (RSD). The single shot-to-shot reproducibility of signals on a single target was +/-19% RSD (n = 7), with no indication of sweet spots or mute spots.