Pulsed Potential Deposition Research Articles

Current-perpendicular-to-plane giant magnetoresistance in Co/Cu multilayered nanocylinders electrodeposited into anodized aluminum oxide nanochannels with ultra-large aspect ratio

An anodized aluminum oxide (AAO) thick film with numerous nanochannels was synthesized by a cathodic exfoliation technique from a metallic aluminum rod. The nanochannel diameter D and length L were about 60 nm and 60 µm, respectively. The aspect ratio L/D reached ~ 1000. A rectangular pulsed potential deposition technique was applied to electrochemically grow the multilayered nanocylinders with alternating Cu and Co thin layers in the AAO nanochannels. The Co/Cu bilayer thickness was decreased to around 7 nm at the minimum by controlling the pulsed potential and duty cycle during the electrodeposition process. An electrochemical in situ single contact process was demonstrated by monitoring the observed current until a Co/Cu multilayered nanocylinder reached the nanoporous Au film on the AAO thick film. The resistance of the Co/Cu multilayered nanocylinder was determined to range from 1 to 2 kΩ, which corresponded well with a theoretical estimation. Current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) of a Co/Cu multilayered nanocylinder with ~ 6000 bilayers (Cu = 2.4 nm, Co = 7.1 nm) reached up to 31.5% at room temperature. Valet–Fert model agreed well with the present study concerning the effect of Cu-layer thickness on CPP-GMR.

Determination of Cobalt Spin-Diffusion Length in Co/Cu Multilayered Heterojunction Nanocylinders Based on Valet-Fert Model.

Anodized aluminum oxide (AAO) nanochannels of diameter, D, of ~50 nm and length, L, of ~60 µm (L/D: approx. 1200 in the aspect ratio), were synthesized and applied as an electrode for the electrochemical growth of Co/Cu multilayered heterojunction nanocylinders. We synthesized numerous Co/Cu multilayered nanocylinders by applying a rectangular pulsed potential deposition method. The Co layer thickness, tCo, ranged from ~8 to 27 nm, and it strongly depended on the pulsed-potential condition for Co layers, ECo. The Cu layer thickness, tCu, was kept at less than 4 nm regardless of ECo. We applied an electrochemical in situ contact technique to connect a Co/Cu multilayered nanocylinder with a sputter-deposited Au thin layer. Current perpendicular-to-plane giant magnetoresistance (CPP-GMR) effect reached up to ~23% in a Co/Cu multilayered nanocylinder with ~4760 Co/Cu bilayers (tCu: 4 nm and tCo: 8.6 nm). With a decrease in tCo, (ΔR/Rp)−1 was linearly reduced based on the Valet–Fert equation under the condition of tF > lFsf and tN < lNsf. The cobalt spin-diffusion length, lCosf, was estimated to be ~12.5 nm.

The Controlled Preparation of Ordered Iridium Oxide Nanotubes Arrays for Oxygen Evolution Reaction

Ordered electrode is believed to be the next generation of MEA. One-dimensional-arrays material possesses high surface area, oriented electron transfer pathway and facile mass transfer pathway, which shows great potential as electrocatalyst. For OER electrode, IrO2 is still the irreplacable choice at present status. To construct IrOx based nanotube/nanowire arrays could increase the performance of OER electrode with reduced loading amount of Iridium. Titanium oxide nanotube arrays (TNTA) is a promising candidate for the preparation of 1-D arrays electrode as its prominent stability, ordered structure and sufficient conductivity with simple modification. In order to achieve the open-end nanotube structure for the fully utilization of noble metal catalyst, controlled electrochemical deposition is applied.In our previous work, IrOx nanotube arrays performs similar current density with 1/20 loading amount of commercial IrO2 nanoparticle at the same potential. To achieve the length control of IrOx nanotube arrays, the electrodeposition of IrOx on TNTA with cyclic voltametry method was studied. By altering the scan rate, continuous IrOx nanotube arrays with different length ranged from 500 nm to 1400 nm was achieved. This phenomenon may probably due to that the deposition in the tube is diffusion controlled, while the deposition at the mouth of TNTA keeps constant due to sufficient supplement of precursor form the bulk electrolyte. In a single cycle, as the depositing goes on, the concentration gradient along the tube forms and results in uneven deposition. Different scan rate results in different duration of depositing in a single cycle. And longer duration means more uneven deposition, and leads to shorter nanotube.At present, to obtain IrOx nanotube arrays with uniform tube wall, control deposition with pulsed potential deposition method is studied. By altering the relaxation and deposition time, IrOx with uniform thickness and tunable length can be achieved. And the thickness of IrOx tube wall can be controlled, too. By achieving uniform and thinner tube wall, the utilization rate of Iridium can be further increased.

CPP-GMR Performance of Electrochemically Synthesized Co/Cu Multilayered Nanowire Arrays with Extremely Large Aspect Ratio.

Anodized aluminum oxide (AAO) films, which have numerous nanochannels ca. 75 nm in diameter, D and ca. 70 µm in length, L (ca. 933 in aspect ratio, L/D), were used as a template material for growing Co/Cu multilayered nanowire arrays. The multilayered nanowires with alternating Cu layer and Co layers were synthesized by using an electrochemical pulsed-potential deposition technique. The thickness of the Cu layer was adjusted from ca. 2 to 4 nm while that of the Co layer was regulated from ca. 13 to 51 nm by controlling the pulsed potential parameters. To get a Co/Cu multilayered nanowire in an electrochemical in-situ contact with a sputter-deposited Au thin layer, the pulsed potential deposition was continued up to ca. 5000 cycles until the nanowire reached out toward the surface of AAO template. Current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) effect reached up to ca. 23.5% at room temperature in Co/Cu multilayered nanowires with ca. 3500 Co/Cu bilayers (Cu: 1.4 nm and Co: 18.8 nm). When decreasing the thickness of Co layer, the CPP-GMR value increased due to the Valet–Fert model in the long spin diffusion limit.

Electrochemical Detection of Dissolved Hydrogen Sulfide By Platinum-Modified Multi-Walled Carbon Nanotubes Sensors

Multi-walled carbon nanotubes (MWCNTs) were synthesized by thermal chemical vapor deposition (CVD) technique on stainless steel substrates. To catalyze the growth of nanotubes, a thin layer of cobalt catalyst nanoparticles was electrodeposited on the substrates under the optimum deposition potential of −1.1 V vs. SCE, leading to the formation of bamboo-shaped MWCNTs. The structural and chemical characterization of carbon nanotubes by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) confirmed the growth of vertically aligned carbon nanotubes with close ends. The fabricated MWCNTs were subjected to a purification treatment by electrochemical oxidation in sulfuric acid solution. Fourier transform infrared (FTIR) spectroscopy verified the formation of oxygenated functional groups (primarily carbonyl and hydroxyl) at the nanotubes sidewalls after the treatment, and the morphological analyses demonstrated the opening of the nanotubes tips blocked by Co nanoparticles. To improve the catalytic activity of the functionalized MWCNT electrodes and boost their electrochemical performance as biosensors, surface modification of the nanotubes was carried out by the potentiostatic and pulse potential electrodeposition of platinum nanoparticles. The optimum operating parameters were determined as E = −0.6 V, t = 100 s and Eon = −0.6 V, Ton = 100 s, Eoff = −0.35 V, Toff = 10 s in the potentiostatic and pulse potential deposition techniques, respectively. The structural characterizations revealed that the Pt/MWCNT electrode prepared by pulse potential deposition possessed a highly uniform and compact structure of more refined Pt nanoparticles as compared to the Pt/MWCNT electrode modified by the potentiostatic electrodeposition technique. The methanol oxidation reaction (MOR) performance of the fabricated electrodes was investigated by the cyclic voltammetry (CV) tests in an electrolyte containing sulfuric acid and methanol, and the pulse-deposited Pt/MWCNT electrode exhibited superior electrocatalytic activity and anti-CO poisoning capability. The performance of the pulse-deposited Pt/MWCNT electrode as an electrochemical sensor for the detection of dissolved hydrogen sulfide was evaluated. The oxidation of sulfide anions on the designed sensor involved the formation of sulfate species, rather than elemental sulfur, through an irreversible diffusion-controlled process, with a sensitivity of 0.632 μA μM−1 cm−2 and a detection limit of 0.26 μM. These data indicate the remarkable capabilities of the fabricated sensor as a promising candidate for numerous applications in water and wastewater treatment technologies.

Template-free electro-synthesis of PbO2 nanorod with chrysanthemum-like array

PbO2 electrode is one of crucial materials in electrochemical field, however, the fabrications of nanostructured PbO2 materials are highly confined to the template method. In this work, the nanostructured PbO2 materials were controllably synthesized by template-free electrodeposition method. It is found that the microstructure of PbO2 material was highly depended on the electrochemical deposition models, i.e., cyclic voltammetry (CV) method, potentiostatic method (PM), pulse potentiostatic method (PPM) and pulse galvanostatic method (PGM). Especially, PbO2 nanorod built by nanoparticles with chrysanthemum-like array synthesized in the absence of template by pulse potential deposition method showed high specific capacitance and good stability for supercapacitor application.

Electrodeposited Fe-Mo-Ni nanowires and Cu-Mo-Fe-Ni alloy nanowire segments

A novel approach of combining templated-assisted electrodeposition, and a subsequent dealloying/displacement reaction for fabricating Cu-Mo-Fe-Ni nanowire segments is introduced. Precursor Fe-Mo-Ni nanowires were electrodeposited from a citrate-boric acid electrolyte using a polycarbonate template with pulse potential deposition. The embedded wires were dipped into an acid, copper citrate electrolyte to promote dealloying/displacement reactions to generate polycrystalline Cu-Mo-Fe-Ni segments.

(Invited) Fundamental Origin for Crack Formation Cr Films Electrodeposited from Cr3+ Solutions

The electrodeposited thin film coatings are materials commonly used today in almost all fields of technological enterprise. Generally, mechanical, chemical and physical properties of electrodeposited films are largely function of electrodeposition methods used in their synthesis (DC, pulse current, pulse potential deposition etc..) [[i]], solution design (additive content, anions, complexing agents, ligands, supporting electrolyte) [[ii],[iii],[iv]] and temperature of the process. One of the main challenges that electrodeposition is facing today in producing the high quality films is their mechanical integrity and conformal coating of the substrates. Often, this goal is difficult to achieve due to appearance of the cracks in the films which compromise their stability and the intended application. The presented work investigates the fundamental origin of crack formation in Cr thin films electrodeposited from Cr3+ solutions. For this purpose, we have developed a methodology for insitu stress measurements and insitu impedance measurements of Cr films, which allowed us to indirectly monitor the crack formation in Cr films during deposition, aging and annealing process. This was of crucial importance to distinguish different processes in the Cr films that contribute to their stress state, mechanical integrity, cracking and change in their electrical properties. The observed growth stress are negative, i.e. compressive with steady state values in the range below -100 MPa. Yet, we have observed a blistering in Cr films and accumulation of different defects on the Cr film surface during deposition which indicated significant amount of dissolved hydrogen either as interstitial impurity or as a part of chemically bonded H as Cr-hydride. Our studies clearly indicate that the tensile relaxation in Cr films after electrodeposition is directly related to the quality of the environment. During relaxation in the solution, the maximum change of stress observed was very modest, just few tens of MPa. The stress data were also complemented with our impedance measurements of Cr films during relaxation in solution which show very modest change in resistance, i.e. disappearance of the hydride phase seems not to be the main driver of the change in electrical properties. However, Cr films aging in air/oxygen shows much greater tensile relaxation and increase in resistance, The insitu stress measurements did not show any direct indication of the crack formation during this process yet, the stress levels observed were becoming close to the values of fracture toughness for pure Cr films[1]. Therefore, presence of oxygen is found as fundamentally important for qualitatively different stress and resistance change of the Cr films during aging process. Our insitu stress, impedance and optical results undoubtedly suggest that annealing process (250 °C) is solely responsible for visible crack formation in Cr films. This is a consequence of the weakened structure of the Cr films by oxidation process which has a true origin in Cr-hydride formation and subsequent oxidation of its degradation by-products (H and Cr). Although the insitu stress data do not show catastrophic failure of Cr films, or brittle fracture propagation that would be obvious signature of the cracking, a closer inspection of the stress data show that their complex behavior in deed represents an interplay of internal structural changes related to chemical processes, as well as formation of micro and macro cracks. The observed levels of tensile stress in some samples are close or exceed the fracture toughness or ultimate tensile strength of the Cr films (>250 MPa)2 at the temperatures of 250 °C. The stress data are in agreement with impedance studies during annealing which also show the largest resistance change. The main conclusion of this work is that the electrodeposition process that produces less Cr-hydride in the deposit should be more desirable when the crack free Cr films are the goal. This work is supported by Gift Grant from NASF-AESF Research Foundation. [1] U. Holzwarth, H. Stamm / Journal of Nuclear Materials 300 (2002) 161–177 REFERENCES [i]) Modern Electroplating V , editors: M. Paunovic and M. Schlesinger, John Willey and Sons, Inc (2010), [ii]) E. I. Cooper, C. Bonhote, J. Heidmann, Y. Hsu, P. Kern, J. W. Lam, M. Ramasubramanian, N. Robertson, L. T. Romankiw and H. Xu, IBM J. Res. & Dev., 49, 103 (2005). [iii]) W. Ehrfeld, Electrochim. Acta, 48, 2857 (2003). [iv]) S.R. Brankovic, N. Vasiljevic, N. Dimitrov, Chapter 27- Applications to Magnetic Recording and Microelectronic Technologies, Modern Electroplating V , editors: M. Paunovic and M. Schlesinger, John Willey and Sons, Inc (2010), p 573.

Pulse Potential Deposition - an Experimental Protocol for Growth of High Quality Thin Films Via Surface Limited Red-Ox Replacement Reaction

Pulse Current Deposition method for growth of thin films and functional coatings has been in use in academia and industry for quite some time [[1]]. However, with few exemptions [[2]], the application of the pulse potential deposition for growth of metallic coatings was mainly confined to academic studies where the conductivity of the substrate, substrate morphology and size do not present a major challenge. Over the years, different deposition protocols have been developed which leverage the benefit of Underpotentially Deposited Monolayers (UPD ML) as a surfactants, mediators or sacrificial materials in deposition process [[3]]. In these examples, the essential step that results in the desired growth mode is achieved by precise control of the electrode potential at which action of the UPD ML is exploited. One of them is deposition via Surface Limited Redox Replacement (SLRR) reaction [[4],[5]], where the main role of UPD layer is to serve as sacrificial reducing metal for deposition of controlled amount of more noble metal. Both, thermodynamics and kinetics of this deposition process are now reasonably well understood which opens more opportunities for practical applications [[6]]. In this talk we show that the control of deposition process during pulse potential deposition is facile and extendable to large area sample plating. The experimental study focuses on design of simple pulse potential function (on/off) for deposition of noble metal coatings on model substrates such as Au and on industrially relevant substrates such as Cu and Ru. The “on” step of the potential cycle is designed to produce a full UPD ML on the substrate surface. The main growth of the deposit occurs during the “off” step of the potential cycle. In this step, at OCP, the deposition of noble metal occurs via SLRR of UPD ML [5]. We will discuss the important parameters of the potential pulse function such as pulse time, off time, and their relation to the deposit morphology, composition and stress. The terminal effect as well as the effect of solution conductivity and solution design in terms of the mutual ratio of UPD metal and depositing metal ion concentration will be discussed as well. [1] J.C. Puippe and F. Leaman, Theory and Practice of Pulse Plating, AESF, (1986). [2] Modern Electroplating V, editors: M. Paunovic and M. Schlesinger, John Willey and Sons, Inc (2010). [3] Chapter 27- Applications to Magnetic Recording and Microelectronic Technologies, S.R. Brankovic, N. Vasiljevic, N. Dimitrov, Modern Electroplating V, editors: M. Paunovic and M. Schlesinger, John Willey and Sons, Inc (2010). [4] S. R. Brankovic , J. X. Wang and R.R. Adzic, Surface Science , 474, L173, (2001). [5] N. Dimitrov, Electrochimica Acta, 209, 599 (2016). [6] Chapter 3: Electrochemical Surface Professes and Opportunities for Material Synthesis, Stanko R. Brankovic and Giovanni Zangari in Electrochemical Engineering Across Scales: From Molecules to Processes, Editors: R. C. Alkire, P. N. Bartlett, J. Lipkowski, Advances in Electrochemical Science and Engineering, vol. 15, Willey-VCH (2015) p. 59-107.

Enhanced Photoelectrocatalytic Activity in Ni-TiO2 Nanocomposites Fabricated By Pulse Potential Deposition

The Ni-TiO2 single layer nanocomposites were fabricated on copper substrates by the pulse plating (PP) and pulse reverse plating (PRP) techniques. Morphological studies conducted by field emission scanning electron microscopy (FESEM) revealed that pulse potentials and durations affected the microstructure of coatings. The chemical composition of coatings was studied by energy dispersive X-ray system (EDX) and it was found that by the prolongation of current-off durations in PP and anodic cycles in PRP, deposits containing 11.58 wt.% and 13.16 wt.% TiO2 were produced, respectively. The influence of heat treatment temperature on the phase structure of Ni-TiO2 nanocomposites was investigated using X-ray diffraction and it was demonstrated that the anatase to rutile transformation occurred at temperatures higher than 450°C. The hydrophilicity of Ni-TiO2 coatings was evaluated by monitoring the change of water contact angle under the UV illumination. It was observed that the nanocomposite prepared by PRP exhibited the maximum hydrophilicity after 3 hours of heat treatment at 450°C, for which the contact angel prior to UV irradiation was 78.96° and after 1h of UV irradiation it reduced to 13.65°. Surface hydroxyl groups density which account for the photo-induced hydrophilicity of the Ni-TiO2 coating was defined by X-ray photoelectron spectroscopy (XPS), before and after 1h of UV illumination. The results indicated that under the optimized condition, for the coatings prepared by PP and PRP, 12.6% and 15.3% increases were detected in the density of hydroxyl groups. To evaluate the effect of pulse plating parameters on the photocatalytic activity of the Ni-TiO2 coatings prepared by PP and PRP, photo-degradation experiments of methyl orange aqueous solution were carried out under the UV light. The results indicated that TiO2 content, irradiation duration and sintering temperature affected the photocatalytic efficiency of the coatings, and the optimized PP and PRP deposited coatings were capable of disintegrating the methyl orange up to 76.21% and 51.89% after 1 hours of UV illumination. The photoelectrocatalytic degradation of methyl orange by Ni-TiO2 coatings was studied and the effect of different applied potentials was determined. The amounts of methyl orange degraded by the PP and PRP deposited composites under the anodic potential of 600 mV along with 1 hours of simultaneous UV illumination were 75.30% and 89.61%, respectively.

Ultra-low Pt loading catalyst layers prepared by pulse electrochemical deposition for PEM fuel cells

We report the production of efficient, ultra-low Pt loading catalyst layers on fuel cell electrodes via pulse potential deposition (PPD) from a H2Pt(OH)6 plating solution. We show that by optimizing the off time in the PPD pulse sequence, it is possible to deposit very small, crystalline Pt nanoparticles with narrow size distribution (1.36 ± 0.360 nm) on amorphous carbon supports. Electrodes fabricated from these high surface area electrocatalysts display increased fuel cell performance compared to commercial electrodes with up to an order of magnitude less Pt. The improved performance of our electrodes is attributed to a combination of the optimized size of the deposited nanoparticles as well as the decreased thickness of the catalyst layer. In our deposited electrodes the catalyst layers are very thin (<10 μm) and as a result achieve up to twelve times the gravimetric power of commercial electrodes.

Controlled Electrodeposition and Magnetic Properties of Co35Fe65 Nanowires with High Saturation Magnetization

Among all transition metals magnetic alloys, Co35Fe65 possesses the highest saturation magnetization BS = 2.45 T at room temperature given by the so-called “Slater-Pauling limit”. For controlled electrodeposition of Co35Fe65 nanowire arrays the following parameters were found to be optimal: electrolyte solution with 1–2 mM malonic acid (MA), ionic ratio Fe+2/Co+2 = 2.0, growth rate, and pulsed potential deposition with time-on (2.5 s) at the potential of −1.15 V/SCE and time-off (1.0 s) at −0.70 V/SCE. These arrays were deposited inside anodic aluminum oxide (AAO) templates that contained columnar nanopores with diameters either 35 or 200 nm. Cyclic voltammetry was used in solution with and without MA and reaction mechanism was proposed to explain the critical role of MA in electrodeposition of CoFe alloys. In addition to uniform deposition of stechiometric Co35Fe65 alloys, a selectivity ratio, (SR) ∼1.0, were achieved, which means that the atomic ratio of Fe/Co in the nanowire matched the molar ratio of Fe+2/Co+2 in the electrolyte. The magnetic behavior of the subsequent 2.45 T Co35Fe65 nanowire arrays showed that the shape and magnetostatic anisotropies dominated the effective anisotropy, and the impact of magnetocrystalline and magnetelastic anisotropies field was very small.

A novel ethanol biosensor on pulsed deposited MnOx-MoOx electrode decorated with Pt nanoparticles

Transition metal oxides provide low cost alternatives to noble metals due to their unique electrocatalytic and electrochromic properties and display strong interactions with noble metal nanoparticles. Present study describes the use of mixed molybdenum and manganese oxide film electrode enriched with platinum nanoparticles to enhance the catalytic performance in the oxygen reduction reaction (ORR). The mixed oxide film, denominated as GCE/MnOx-MoOx/Pt, was co-deposited by pulsed potential deposition technique and then, it was used as an immobilization matrix for the intact bacterial cells. The resulted whole cell biosensor has served as a sensing platform for ethanol detection by monitoring of oxygen consumption as a result of the bacterial metabolism in the presence of the substrate. Following the optimization experiments, GCE/MnOx-MoOx/Pt/Gluconobacter oxydans biofilm was applied to investigate the analytical characteristics and response time to ethanol as well as operational stability and substrate specificity for other carbon sources. The linear range was found as 0.075–5.0mM with a response time of 63s and the developed method was applied to the ethanol determination in alcoholic drinks. Satisfactory recovery figures were obtained in comparison to the standard method.

Pulse potential deposition of thick polyvinylpyridine-like film on the surface of titanium nitride

We achieved covalent attachment of a thick polyvinylpyridine-like polymeric film on a 200 mm diameter titanium nitride wafer (MEMS industry standard) using a simple electrochemical setup.

TEM investigations on the local microstructure of electrodeposited galfenol nanowires

The local microstructure of Fe–Ga nanowires is investigated considering dependence on the deposition technique. Using a complexed electrolyte, smooth and homogeneous Fe80Ga20 nanowires are deposited into anodic aluminum oxide templates by either applying pulse potential or potentiostatic deposition technique. At optimized deposition conditions the wires show the desired composition of Fe80±2Ga20±2 without a gradient along the growth direction. Composition distribution, structure and microstructure are examined in detail and reveal only minor differences. Line EELS and crystal lattice measurements reveal a negligible oxygen content for both preparation routines. Neither Fe/Ga oxides nor hydroxides were found. Both potentiostatically deposited as well as pulse deposited nanowires exhibit a preferred 〈110〉 orientation, the latter with slightly larger crystals. Different contrast patterns were found by TEM that appear more pronounced in the case of pulse deposited wires. High resolution transmission electron microscopy analysis and comparison of differently prepared focused ion beam lamellas reveal that these contrasts are caused by defects in the alternating potential deposition itself and are not induced during the TEM preparation process. The alternating potential mode causes periodic growth thereby inducing different layers with reduced wire thickness/defects at the layer interfaces.

Fabrication of CdTe/ZnS core/shell quantum dots sensitized TiO2 nanotube films for photocathodic protection of stainless steel

A highly ordered TiO2 nanotube array film on a titanium substrate was fabricated by an anodic oxidation method. Then CdTe/ZnS core/shell quantum dots were successfully prepared on the film by pulsed potential electrodeposition and chemical bath deposition in succession. The results indicated that the CdTe/ZnS quantum dots sensitized TiO2 composite film displayed a significant increase in photocurrent compared with a pure TiO2 nanotube film, and its photoabsorption was extended to the visible region. Under white light illumination, the composite film as a sustainable photoanode had better photoelectric conversion performance and could provide striking photocathodic protection for 403 stainless steel.

Ultrathin Platinum Films for Methanol and Formic Acid Oxidation: Activity as a Function of Film Thickness and Coverage

Self-terminating electrodeposition was used to grow ultrathin Pt overlayers on 111 textured Au thin films. The Pt thickness was digitally controlled by pulsed potential deposition that enabled the influence of overlayer thickness on electrocataytic reactions, such as methanol and formic acid oxidation, to be examined. Bimetallic and ensemble effects associated with submonolayer coverage of Pt on Au yield enhanced catalysis. For films grown using one deposition pulse, the peak rate of CH3OH oxidation was enhanced by a factor of 4 relative to bulk Pt. The overlayer consisted of 2 nm diameter monolayer Pt islands that covered 75% of the surface; however, voltammetric cycling resulted in a loss of the enhanced activity associated with the as-deposited submonolayer films. For thicker Pt films, the electrocatalytic activity decreased monotonically with thickness until bulk Pt behavior was obtained beyond three monolayers. For HCOOH oxidation improvements in the Pt area, normalized activity in excess of a 100-fold were observed for submonolayer Pt films. The performance improved with voltammetric cycling as a result of a combination of Pt dissolution, Au segregation, and Pt–Au alloy formation. The maximum activity was associated with fractional surface coverage between 0.28 and 0.21, although the films were subject to a deactivation process at longer times related to a diffusional process. Bulk Pt behavior for formic acid oxidation was observed for Pt films greater than three monolayers in thickness.

Electrografted diazonium salt layers for antifouling on the surface of surface plasmon resonance biosensors.

Electrografted diazonium salt layers on the surface of surface plasmon resonance (SPR) sensors present potential for a significant improvement in antifouling coatings. A pulsed potential deposition profile was used in order to circumvent mass-transport limitations for layer deposition rate. The influence of number of pulses with respect to antifouling efficacy was evaluated by nonspecific adsorption surface coverage of crude bovine serum proteins. Instead of using empirical and rough estimated values, the penetration depth and sensitivity of the SPR instrument were experimentally determined for the calculation of nonspecific adsorption surface coverage. This provides a method to better examine antifouling surface coatings and compare crossing different coatings and experimental systems. Direct comparison of antifouling performance of different diazonium salts was facilitated by a tripad SPR sensor design. The electrografted 4-phenylalanine diazonium chloride (4-APhe) layers with zwitterionic characteristic demonstrate ultralow fouling.

Electrodeposition of Pt Nanoparticle Catalysts from H2Pt(OH)6 and Their Application in PEM Fuel Cells

We report the electrochemical deposition of Pt nanoparticles from platinic acid H2[Pt(OH)6] using pulse potential deposition (PPD). We are able to control the size, morphology, and loading of platinum nanoparticles from H2[Pt(OH)6] by controlling the deposition parameters such as the pH of the plating solution, the pulse potential, the pulse width, and the duty cycle of the pulse sequence. We show that a high density of Pt nanoparticles electrodeposited can be produced on both planar and nonplanar electrode supports with high surface area and high catalytic activity. Finally, we show that fuel cell electrodes can be produced using H2[Pt(OH)6] as the source of Pt via our optimized PPD technique. The fuel cells produced from these electrodes are highly efficient with less than half the Pt content of commercially available fuel cells, which results in a gravimetric power more than twice that of fuel cells produced by using commercially available electrodes.

Characterization of electrografted 4-aminophenylalanine layers for low non-specific binding of proteins

Novel diazonium salts based on the 4-amino derivative of phenylalanine were electrografted onto gold surfaces with the ultimate goal of formation of surfaces resistant to nonspecific adsorption of proteins. A pulsed potential deposition profile was used instead of the more conventional approaches in order to circumvent mass-transport limitations. The influence of the deposition parameters, including pulse potentials, pulse width and number of pulses, with respect to electrode coverage was evaluated as a function of the blocking effect towards the diffusion of Fe(CN)64−/3− to electrode surfaces. By appropriately choosing the deposition parameters, peak potential differences (ΔEp) of ∼750 mV (for ν = 100 mV s−1), contrasting the ∼440 mV obtained for layers deposited via conventional electrochemical methods. FT-IR spectra confirm that the general structure of the electografted layers displays the same chemical functionality as the precursor molecule. Furthermore, the presence of the carboxylic acid group, characterized by the absorption feature at 1724 cm−1, indicates that the layers retain the ability to undergo post-deposition functionalization with bioreceptors. Ellipsometric analysis demonstrates the versatility of this method by depositing layers ranging from ∼1 to ∼24 nm thick. Finally, surface plasmon resonance spectroscopy was used to monitor the modified surfaces upon exposure to highly fouling media (76 mg mL−1 bovine serum albumin in phosphate buffered saline solution). A protein surface coverage equivalent to 62 ng cm−2 was measured, representing a significant improvement compared with more established antifouling layers based on polyethylene glycol (100 ng cm−2) and alkanethiol self-assembled monolayers (268 ng cm−2). The resistance towards nonspecific adsorption may be associated with the hydration layer tightly bound by the ionic charges present in the organic layer at physiological pH.