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Synthesis of nano-composite surfaces via the co-deposition of metallic salts and nano particles
Materials Science and Engineering B 182 (2014) 59–68
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis of nano-composite surfaces via the co-deposition of metallic salts and nano particles J.W. Mac Farlane ∗ , S.J. Tesh, R.A. Crane, K.R. Hallam, T.B. Scott Interface Analysis Centre, Oldbury House, 121 St. Michael’s Hill, Bristol BS2 8BS, United Kingdom
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Article history: Received 20 May 2013 Received in revised form 28 October 2013 Accepted 13 November 2013 Available online 26 November 2013 Keywords: Electro-deposition Nano-composite Iron nanoparticles Water remediation Synthesis
a b s t r a c t A novel, low energy method for coating different nano-particles via electro-deposition to a recyclable carbon glass supporting structure is demonstrated. In the resulting composite, the nano-material is bound to the substrate surface, thereby removing the potential for causing harmful interactions with the environment. Nano-particles were suspended in a salt solution and deposited at low current densities (<0.1 A cm−2 ) producing thin (<100 nm), uniform nano-faceted surfaces. A co-deposition mechanism of nano-particles and cations from the salt solution is proposed and explored. This has been successfully demonstrated for iron, sliver, titanium in the current work. Furthermore, the removal of the surface coatings can be achieved via a reversed current applied over the system, allowing for the recovery of surface bound metal contaminants. The demonstrated applicability of this coating method to different nano-particle types, is useful in many areas within the catalysis and water treatment industries. One such example, is demonstrated, for the treatment of BTEX contamination and show a greatly improved efficiency to current leading remediation agents. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, nano-particles of varying compositions have been demonstrated as effective for the sterilisation of harmful bacterial [1,2] and removal of harmful pollutants from water [3–9]. However, whilst nano-materials may well provide an effective new tool for the clean-up of pollutants they will not be universally adopted until deployment methods can be developed that limit or remove these aforementioned risks relating to nanomaterial toxicity [10,11]. The fabrication of nano-composites is an obvious way of achieving this and forms the basis for the research presented here in. Whilst the limited existing literature on this topic has demonstrated the general feasibility of nano-composites for water treatment purposes, there are numerous fundamental flaws in the design of the composites developed. These include: (i) lack of structural performance, (ii) reduction in nano-material reactivity, (iii) use of surfactants and (iv) limited sustainability [12–16]. The method presented in the current work specifically addresses these key design limitations to develop a new suite of nano-composite materials, whereby the support material imparts the mechanical properties of the composite and at the same time the desired physical coating of well-bound nano-material, imbues the composite with nano-reactivity.
∗ Corresponding author. Tel.: +44 1173317683. 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.11.011
The nano-composite fabrication method represents a simple but intuitive adaptation of standard electro-deposition techniques. Electro-deposition is a well established method [17], in which an electronic field is applied over a salt solution, with deposition occurring on the target electrode surface. In the automotive industry, this technique has been refined for high current (3 A cm−2 ) deposition of galvanising coatings [18]. Whilst some previous studies have examined the use of electro-deposition and co-deposition of nano-scale material, their focus has been on forming coatings between 50 and 100 m thick, with nano-particles embedded throughout the structure, without an overall nano-scale surface [13]. Furthermore, the applications have been focused around photonics, batteries and sensors [19–23] and seemingly little research has been directed at developing electro-deposited nano-structures for water remediation applications. Here we present a modification on the technique, using a significantly lower current density (0.09 A cm−2 ) for the deposition and formation of nano-composites, using a colloidal suspension of nano-particles in a metal salt solution. In this method the electrostatically bound nano-particles act to provide low energy sites for surface nucleation of nano-scale metallic crystallites induced by electrochemical reduction of the aqueous metallic salts [24]. The resulting process is one of codeposition and heterogeneous crystal growth, all at the nano-scale. The demonstrated method is considered to provide a synthesis route that is greener, cleaner and more readily usable for multiple nano-composites with beneficial applications in water and air
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Fig. 1. The deposition of nano-composite surfaces, formed from an Fe salt solution containing suspended INPs; top left nano-composites showing mixed cubic and spherical structures. Top right, focused ion beam (FIB) cross section demonstrating the deposited surface thickness The nano-composite is seen between the boxed region with a protective layer of Pt deposited above the surface and the substrate below. Bottom left, scanning transmission electron microscopy image of part of the nano-composite coating, illustrating the formation of cubic crystals over spherical nano-particle centres. Bottom right atomic force microscopy (AFM) data of a 5 m2 region.
filtration (industrial and domestic), and the potential for alternative uses in other areas of industry. By applying the coating method to a substrate of reticulated vitreous carbon (RVC) foam, a highly porous nano-composite suitable for both in situ and ex situ water filtration is created. The current work demonstrates how a reactive and nano-structured coating may be created throughout the filter and investigates the physiochemical benefits for the iron nanoparticles (INPs) of post synthesis vacuum heat treatment, which has previously been shown to improve the structure, and hence reactivity, of free INPs [25,26,28,29]. Data presented here demonstrates the arising filter material to be highly effective for the remediation of a range of volatile aromatic organic pollutants at rates and magnitudes competitive with well-established remediation agents; organoclay (OC) and granulated activated carbon (GAC).
the INP material. Approximate surface area was determined from the AFM data, using a surface area integration method [29], to be 3 ± 0.5 m2 g−1 of surface coating. X-ray photo-electron spectrometry (XPS) was used to confirm the oxidation state of the formed nano-composite surfaces, the presence and chemistry of surface contaminants (Fig. 2). The analysis confirmed the presence of both metallic iron and iron oxide in the surface analysis volume. With an approximate maximum sample depth of 6 nm (for the XPS analysis conditions) the recorded oxide signal is ascribed to the formation of a (<2 nm, Fig. 1 TEM image) surface oxide layer (<2–3 nm) on the deposited metallic
2. Results and discussion 2.1. Deposition of nano-composite iron surfaces Results are shown of a coating formed from an Fe salt solution containing suspended INPs (Fig. 1). Under empirically determined deposition variables nano-faceted surface coatings of metallic iron were successfully and repeatably deposited on vitreous carbon substrates, as shown in Fig. 1. The AFM and SEM data show the resulting nano-texture of the coatings to include both cubic and spherical structures. Using focused ion beam (FIB) cross-sectioning (Fig. 1), the surface coating was determined to between 50 and 100 nm in thickness; approximately equivalent to the average diameter of
Fig. 2. XPS of Fe 2p3/2 peaks of the nanocomposite surface formed from an Fe salt solution containing suspended INPs.
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iron coating. Spectrum curve fitting results of the recorded Fe2p photoelectron profile indicated an Fe/FeO oxide ratio of 0.1627 and Fe(II)/Fe(III) = 0.539. Consequently the inferred oxide stoichiometry is that of a near stoichiometric magnetite (Fe3 O4 ) grown on the metallic iron nano-coating, after formation of the composite This is in agreement to previously documented studies [26,28]. When considering the influencing parameters on the electrodeposition method a large number of variables were considered, examples images from each of the variables can be found in the appendix. Of all the parameters the deposition current was considered the most important for surface formation; a greater current induced a larger flux of ions towards the cathode [30], but also caused a more rapid evolution of hydrogen (from the generation of aqueous H+ ions). The production of H2 bubbles at the cathode prevented the metallic surface from binding to the substrate. Consequently the effect of current density was explored over a range of values and the resulting coverage of iron was closely examined by SEM and energy-dispersive X-ray spectroscopy (EDX). The most appropriate value was found to be approximately 0.09 A cm−2 . Electro-deposition time was considered another important factor; longer deposition times created a thicker coating layer; however, this rapidly became uneven and began to spall from the electrode surface. The optimum time period was identified through similar iterative testing but using SEM analysis and focused ion beam sectioning. Sonification of the electrolyte was performed to encourage the nano-particles to remain suspended in the solution and minimised sedimentation and aggregation; however, it simultaneously prevented the nano-particles from anchoring to the substrate and reduced crystallisation of the Fe2+ ions. Resultantly, a pulsed plating method was adopted whereby pulses of current were alternated with pulses of sonification, allowing the INPs to anchor to the substrate whilst concurrently preventing sedimentation. To obtain a thicker, uniform coating layer the method was further developed by using a reverse-plating technique. This technique adaptation reverses the polarity of the electrodes at regular intervals in order to disperse uneven accumulations of the nano-coating. Various time intervals were tested for electrode reversal and the results were studied via SEM to identify the best conditions. Whilst iron based systems were of primary interest the applicability of the coating process to other material systems was also investigated, specifically the Ag/AgN, TiO2 /AgN and Pt/PtCl mixed nano-particle:electrolyte systems were examined, Fig. 3. In the Ag/AgN system the deposition of similar nano-faceted surfaces coatings was readily achieved (Fig. 3). However due to the propensity for Ag to form small nano-crystals during deposition, silver dendrites were observed to form on top of the original (and somewhat larger) silver nano-particles. In this case the silver nanoparticles used were relatively large to illustrate that the particles act as nuclei for crystal formation from solution (dendritic structures shown in Fig. 3). For the TiO2 nano-particle:AgN salt system a highly structured nano-composite coating was readily formed and demonstrates the possibility of mixing different nano-particles types with different metals salts to form alternative composites. Due to the size of the particles, it is not necessary to use a permanently charged particle to be influenced by the electric field. TiO2 particles have been cemented together with silver salt deposited out of solution, and show smaller crystal grains growing on the surface of the particles. For the Pt/PtCl system results observed were very similar to that of the INP/Fe system. For a mixed phase system (containing PtNP/INP and corresponding salts) a uniform structure was also observed. However the deposition rate was greatly accelerated in comparison to the monometalic systems, with crystallisation of metallic Pt acting to rapidly cement the voids between particles. For the same coating periods as used for the INP/Fe system, coatings approximately 5 times thicker were formed which have
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Fig. 3. Nano-composites formed from difference deposition systems, with Ag nano-particles AgN salt (top left), Pt nano-particles PtCl3 salt (top right) TiO2 nanoparticles AgN salt (bottom left) Pt nano-particles FeCl3 salt (bottom right).
a significantly reduced surface area relative to other non-mixed metal systems. In all the systems studied the particle concentration was found to have a significant bearing on the nano-composites formed. This is evidenced by examining end members of the deposition concentrations, both suspensions of nano-particles and a pure salt solution. With each end member a different structure is formed (see Fig. 4). With a pure INP solution resulted in the generation of poorly distributed agglomerated masses of INP on the surface. This is ascribed to the particles acting as points of charge concentration with subsequent particles attracted to these point structures and resulting in accumulations of particle agglomerates tens to hundreds of microns in extent. Limited dissolution of the INP was considered to potentially provide a finite source of aqueous metal ions to cement the agglomerates as they formed. By comparison a salt only system deposited a smooth even metallic Fe layer with no
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Fig. 4. Nano-composites formed from an Fe salt solution containing suspended INPs illustrating the differences between a pure salt deposition (top left), a pure nanoparticle deposition (top right), and idealised proportions (50:50) of nanoparticles to salt in the solute (bottom left, bottom right).
nano-faceted surface structure. This was ascribed to the comparative lack of nucleation sites provided in the other systems by the nano-particles. Consequently nano-faceted coatings were only observed when the electrolyte contained both an aqueous salt and suspension of a significant quantity of nano-particles. It is proposed that the nano-particles act as nucleation sites for electrodeposition of the aqueous metallic salts [30]. By establishing low starting pH for the electrolyte, below that of the point of zero charge (PZC) of the nanoparticle surface, H+ ions functionalised the suspended particles, giving a net positive surface charge. It is proposed that the locally suspended INP are attracted to the cathode surface via an electrical charge effect. On contact with the surface the nano-particles subsequently act as seed-nuclei acting to lower the activation energy
for electrodeposition of the metal cations from the solution. This can be seen in the high resolution STEM images (Fig. 1), with nanoparticles shown at the substrate:layer interface. This mechanism is demonstrated to apply to both the metallic and non-metallic nano-particles examined here. It is considered that this mechanism is key to the deposition process with the significant number density of surface particles (estimated at 3 × 1013 cm2 ), preventing the electrodeposition of featureless metallic iron thin films on the conductive substrate. Although the surface charge resulting from the low pH allows the nano-particle to be influenced by the electric field the attraction is assumed to be relatively short range. This means that only nano-particles close to the surface of the substrate can actually be attracted, fixed and therefore can act as nucleation sites for the
Fig. 5. Nano-composites formed from an Fe salt solution containing suspended INPs and illustrates the surface of the nano-composite at the top of the formed filter material (right) and middle of the filter material (left).
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Fig. 6. The percentage of organic removed normalised by surface area of reactive material (INC 12 m2 g−1 , GAC 65 m2 g−1 , OC 825 m2 g−1 , determined by BET analysis).
deposition. To this end the pulsed sonification allows the particles to remain suspended throughout the fluid in an even concentration, and not depleted in the bonding region due to the deposition of nano-particles on the surface. Furthermore sonification prevents the aggregation and clumping of nano-particles within system, without the use of surfactants or other chemicals that would alter the surface chemistry of the nano-particle and would normally be used to prevent aggregation. Sonification also has the added benefit of desorbing H2 bubbles from the surface of the cathode. Pulsed sonification was found to display the best results as interval pulsing was sufficient to prevent nano-particle aggregation, without resulting in degradation of the surface coating, as continual sonification was observed to remove nano-particles loosely bound to the substrate surface. The variable found to be key to the uniformity and repeatability of the coating process was the applied current density. The current density had to be high enough to draw the ions to the surface; however, at low pH H+ ions are also drawn to the cathode and result in the generation of H2 gas. This evolution of gas
prevents the formation of a stable, regular surface. Consequently at high current densities (>1 A cm−2 ) the formed surfaces are cracked and inconsistent with bubble like structures observed in the coatings. Even at low current densities, polarity reversal of the electrodes was necessary to mitigate the rapid build-up of materials at points of charge concentration. This method was found to provide repeatably uniform surface coatings (thickness and morphology).
2.2. Water remediation The refined electrodeposition process creates a nano-structured layer on the surface of the carbon throughout the porous structure (Fig. 5). The coating consists primarily of quasi spherical features, approximately 20–50 nm in size, created by the deposition of INPs cemented to the surface by the dissociated iron from FeCl3 . These features are interspersed by faceted metallic crystals, resulting
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Fig. 7. A schematic diagram of experimental setup.
Fig. 8. A schematic diagram of experimental setup for filter production.
from the INPs acting as nucleation sites for heterogeneous crystal growth. The study compared the performance of OC, GAC and the iron nano-composite (INC) in remediating BTEX organic compounds (benzene, toluene, ethyl-benzene and m-, p-, o-xylene). The results from ChemTest Ltd. demonstrated that the INC exhibited BTEX uptake at rates and magnitudes very closely comparable with OC and GAC. However, once this data is normalised by the surface area of the reactive material, it becomes clear that the INC significantly outperforms the comparator materials (Fig. 6). Based on other INP studies [7] the mechanism involved for BTEX removal onto the INC developed is considered to be adsorption followed by degradation caused by the organic-iron coordination weakening of the benzene ring. This is compared to OC and GAC which remove organics via adsorption alone [35,36]. The result of this investigation implies that the INC is more reactive than traditional agents, having beneficial implications for both cost and resources required for remediation. Further work will explore the application of this coating method to different sectors of water remediation including heavy metals, radionuclides and complex organic molecules.
3. Conclusions We present here a novel technique for the anchoring of nanoparticles on to a conductive substrate to form a nano-composite. The technique has been demonstrated as highly effective at binding both metallic and non-metallic nano-particles with a variety of aqueous metallic salts systems, including NP/salt systems of: Fe/FeCl3 , Ag/AgN, Pt/PtCl3 and TiO2 /FeCl3 . The nano-composites formed are observed to have a uniform, nano-faceted surface covering centimetre sized areas of the substrate. The key conclusions here presented are that a variety of metallic and non-metallic nano-particles have been deposited on a conductive carbon glass substrate including NP/salt systems of: Fe/FeCl3 , Ag/AgN, Pt/PtCl3 and TiO2 /FeCl3 . The formed nano-faceted surfaces show highly enhanced reactive surface areas (3 ± 0.5 m2 g−1 of surface coating). Current density (<0.1 A cm−2 ) and particle concentration (50:50 salt:NP) used for deposition were determined as the most important factors for successful deposition. However, sonification and reverse plating of the substrate offered further improvements. The substrates can be re-cycled by applying a reverse current, which removes the surface coating. The deposition technique is low technology, low energy and does not
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Fig. 9. Secondary electron micrograph images demonstrating the effect of the deposition variables on the deposited surface, formed from an INP FeCl3 system on a reticulated foam. The subfigures explore: deposition time (a) 10 min, (b) 25 s; FeCl3 :INPs ratio, (c) 1:3, (d) 1:1; substrate porosity, (e) 20 ppi, (f) 30 ppi; electrodeposition current, (g) 0.1A, (h) 0.2A, (i) 0.2A, (j) 0.3A; pulsed current (outside surface of foam), (k) no pulsed plating, (l) pulsed plating; pulsed current (centre of foam), (m) no pulsed plating, (n) pulsed plating; reverse current pulses, (o) forward current pulses, (p) forward and reverse current pulses; pulse length (q) 60 s forward, 10 s reverse (r) 20 s forward, 10 s reverse.
require the use of expensive precursors, and is considered applicable to a large range of nano-particle types. A demonstration of the INP/FeCl3 nanocomposites as a water filter is presented, which out performs current market leading materials. 4. Experimental 4.1. Nano-particles synthesis Nano-structures were manufactured in the present work using nano-particles of metallic iron (INP), titanium dioxide (TiO2 ), platinum (Pt) and silver (Ag). The materials were selected for their well-documented reactivity and respective capabilities for pollutant remediation. They represent the nano-materials most commonly used on a commercial basis and may be readily synthesised or purchased. For example, INPs are used for environmental remediation purposes as they are: (i) highly reactive, (ii) low cost,
(iii) available in kg quantities, (iv) environmentally compatible, and (v) form no hazardous by-products. Bi-metallic iron–nickel nano-particles, (BNP), were synthesised following an adaptation of the method first described by Wang and Zhang [31], using sodium borohydride to reduce ferrous Fe to a metallic state. Briefly, 7.65 g of FeSO4 7H2 O was dissolved in 50 ml of Milli-Q water (18.2 M cm) and then a 4 M NaOH solution was used to adjust the pH to the range 6.2–7.0. The salts were reduced to metallic nano-particles by the addition of 3.0 g of NaBH4 . The product was isolated through centrifugation and then sequentially washed with water, ethanol and acetone (20 ml of each). The nano-particles were dried in a desiccator under low vacuum (10−2 mbar) for 48 h and then stored in a nitrogen-filled glovebox (Saffron Scientific) until required. Platinum nano-particles were formed via a modified [31] method, in which, 0.017 g PtCl4 (IV) (Sigma Aldrich) was dissolved
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Fig. 9. (Continued )
in 100 ml of Milli-Q water. The Pt salt solution was added dropwise to a 0.002 mol ml solution of NaBH4 solution under constant agitation. The arising nano-slurry was centrifuged and the supernatant decanted. The residual solid was washed in Milli-Q water, before further centrifugation and the final residual solid was stored with in a vacuum desiccator (>10−2 mbar). Silver nano-particles were also formed in the same way to the Pt nanoparticles using the [31] method; however, 0.017 g AgN (IV) (Sigma Aldrich) dissolved in 100 ml of Milli-Q water as the starting salt solution. TiO2 nano-particles were sourced commercially (Degussa P25). The metallic salts used (FeCl3 , AgN, PtCl4 ) were all of analytical grade (Fisher Chemicals). 4.2. Material selection In the present study INP were selected for the most significant analytical characterisation due to its widespread use compared to
the aforementioned nanomaterials [32–34]. Furthermore BNP was selected to use nickel as a tracer, for locating where the particles are deposited and allows them to be distinguished from Fe deposited from an aqueous ionic state. As there is no nickel present within the aqueous solution, the detection of nickel clusters by spectrometry techniques indicates material deposited as nano-particles, and not from the aqueous salt solution, which forms the cementing matrix. 4.3. Deposition method A pre-fabricated vitreous carbon block was used as a conductive substrate (Sigradra, HTW Hochtemperatur Werkstoffe GmbH) onto which the nano-particles were anchored via electrodeposition. The carbon glass acted as the cathode, and a stainless steel beaker was used as the anode and containing vessel for the electrolyte solution (Fig. 7). The electrolyte solution (approximately pH 3 throughout) contained Milli-Q water with appropriate concentrations and ratios
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Fig. 9. (Continued )
of metallic salt and nano-particles. The steel beaker was sonicated in order to keep the nano-particles in suspension and with minimal aggregation. The optimum concentrations were identified after an iterative programme of empirical testing. For deposition on to a vitreous carbon block, optimal deposition was found when a concentration of 5 g L−1 nanoparticle (i.e. INP, TiO2 , Pt, Ag) with 5 g L−1 of appropriate salt (i.e. FeCl3 , AgN, PtCl4 ) was used. 4.4. Filter formation In order to produce a water filter the above deposition method was modified. The substrate discs, 9 mm thick and 47 mm in diameter, of reticulated vitreous carbon (RVC) foam (ERG Aerospace, U.S.A.) with a porosity of 45PPI, were washed ultrasonically in acetone and Milli-Q water as a preparatory cleaning step. The RVC foam discs were maintained in contact with a stainless steel plate forming the cathode. A stainless steel beaker acted as the anode and container for the electrolyte solution, which was a combination
of 0.4 g FeCl3 (99.9% Sigma) and 1 g INPs (NanoIron Ltd., Czech Republic) in 200 ml deionised water, solution pH was not modified and was measured as pH 4. The beaker was held within a sonic bath to ensure the suspension of the INPs (Fig. 8). Before addition of INPs, the electrolyte solution was purged of oxygen using argon to limit the aqueous oxidation of the INPs. The RVC disc was sonicated in the electrolyte before beginning the deposition to allow the INPs to penetrate the foam. A 9 min electrodeposition cycle was performed comprising repeating sets of an initial 40 s period consisting of alternate 5 s bursts of sonication and then current pulses, followed by a 20 s period of alternating 5 s bursts of sonication and reverse current pulses. The foam was then turned over and the cycle repeated. The samples were then rinsed with acetone to remove any excess INPs and placed immediately in a vacuum desiccator to dry. Control samples were formed using electrolyte solutions void of nanoparticles (0.0086 mol FeCl3 ). Resulting nano-structures were analysed using scanning electron microscopy and also using atomic force microscopy.
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A series of samples were vacuum annealed at 600 ◦ C for 24 h at 10−5 mbar. X-ray photoelectron spectroscopy was performed both before and after the vacuum annealing to investigate the change in surface physiochemistry. Organic contaminant sorption tests were performed by ChemTest Ltd. to compare the remedial abilities of the sorbents granular activated carbon (GAC), organo-clay (OC) and the vacuum annealed nano-composite. A groundwater sample was abstracted from a contaminated site in Portsmouth, UK, containing benzene, toluene, ethyl-benzene, and m-, p-, o-xylene (the BTEX group). Kinetic experiments were conducted using continuously stirred 1000 ml glass sealed reactors at a suspension density of 0.4 g L−1 . Samples were taken at time points (t) = 5, 10, 30, 90, 360, 1440 min. Solution chemistries were determined by headspace gas chromatography mass spectrometry for volatile organic compounds. Acknowledgements The authors thank Chris Jones for TEM preparation and to Julia Greenwood for laboratory assistance in this project. Appendix A. Fig. 9. References [1] J.W. MacFarlane, H. Jenkinson, T. Scott, Applied Catalysis B: Environmental 106 (2011) 181–185. [2] M. Redetic, Journal of Materials Science 48 (1) (2011) 95–107. [3] A. Ghauch, A. Tuqan, H.A. Assi, Environmental Pollution 157 (5) (2009) 1626–1635. [4] Z. Fang, J. Chen, X. Qiu, X. Qiu, W. Cheng, L. Zhu, Desalination 268 (13) (2011) 60–67. [5] H. Tian, J. Li, Z. Mu, L. Li, Z. Hao, Separation and Purification Technology 66 (1) (2009) 84–89. [6] S. Klimkova, M. Cernik, L. Lacinova, J. Filip, D. Jancik, R. Zboril, Chemosphere 82 (8) (2011) 1178–1184. [7] R.A. Crane, T.B. Scott, Journal of Hazardous Materials 211–212 (0) (2012) 112–125. [8] P. Calcagnile, D. Fragouli, I.S. Bayer, G.C. Anyfantis, L. Martiradonna, P.D. Cozzoli, R. Cingolani, A. Athanassiou, ACS Nano 6 (6) (2012) 5413–5419.
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Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Synthesis of nano-composite surfaces via the co-deposition of metallic salts and nano particles J.W. Mac Farlane ∗ , S.J. Tesh, R.A. Crane, K.R. Hallam, T.B. Scott Interface Analysis Centre, Oldbury House, 121 St. Michael’s Hill, Bristol BS2 8BS, United Kingdom
a r t i c l e
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Article history: Received 20 May 2013 Received in revised form 28 October 2013 Accepted 13 November 2013 Available online 26 November 2013 Keywords: Electro-deposition Nano-composite Iron nanoparticles Water remediation Synthesis
a b s t r a c t A novel, low energy method for coating different nano-particles via electro-deposition to a recyclable carbon glass supporting structure is demonstrated. In the resulting composite, the nano-material is bound to the substrate surface, thereby removing the potential for causing harmful interactions with the environment. Nano-particles were suspended in a salt solution and deposited at low current densities (<0.1 A cm−2 ) producing thin (<100 nm), uniform nano-faceted surfaces. A co-deposition mechanism of nano-particles and cations from the salt solution is proposed and explored. This has been successfully demonstrated for iron, sliver, titanium in the current work. Furthermore, the removal of the surface coatings can be achieved via a reversed current applied over the system, allowing for the recovery of surface bound metal contaminants. The demonstrated applicability of this coating method to different nano-particle types, is useful in many areas within the catalysis and water treatment industries. One such example, is demonstrated, for the treatment of BTEX contamination and show a greatly improved efficiency to current leading remediation agents. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, nano-particles of varying compositions have been demonstrated as effective for the sterilisation of harmful bacterial [1,2] and removal of harmful pollutants from water [3–9]. However, whilst nano-materials may well provide an effective new tool for the clean-up of pollutants they will not be universally adopted until deployment methods can be developed that limit or remove these aforementioned risks relating to nanomaterial toxicity [10,11]. The fabrication of nano-composites is an obvious way of achieving this and forms the basis for the research presented here in. Whilst the limited existing literature on this topic has demonstrated the general feasibility of nano-composites for water treatment purposes, there are numerous fundamental flaws in the design of the composites developed. These include: (i) lack of structural performance, (ii) reduction in nano-material reactivity, (iii) use of surfactants and (iv) limited sustainability [12–16]. The method presented in the current work specifically addresses these key design limitations to develop a new suite of nano-composite materials, whereby the support material imparts the mechanical properties of the composite and at the same time the desired physical coating of well-bound nano-material, imbues the composite with nano-reactivity.
∗ Corresponding author. Tel.: +44 1173317683. 0921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.mseb.2013.11.011
The nano-composite fabrication method represents a simple but intuitive adaptation of standard electro-deposition techniques. Electro-deposition is a well established method [17], in which an electronic field is applied over a salt solution, with deposition occurring on the target electrode surface. In the automotive industry, this technique has been refined for high current (3 A cm−2 ) deposition of galvanising coatings [18]. Whilst some previous studies have examined the use of electro-deposition and co-deposition of nano-scale material, their focus has been on forming coatings between 50 and 100 m thick, with nano-particles embedded throughout the structure, without an overall nano-scale surface [13]. Furthermore, the applications have been focused around photonics, batteries and sensors [19–23] and seemingly little research has been directed at developing electro-deposited nano-structures for water remediation applications. Here we present a modification on the technique, using a significantly lower current density (0.09 A cm−2 ) for the deposition and formation of nano-composites, using a colloidal suspension of nano-particles in a metal salt solution. In this method the electrostatically bound nano-particles act to provide low energy sites for surface nucleation of nano-scale metallic crystallites induced by electrochemical reduction of the aqueous metallic salts [24]. The resulting process is one of codeposition and heterogeneous crystal growth, all at the nano-scale. The demonstrated method is considered to provide a synthesis route that is greener, cleaner and more readily usable for multiple nano-composites with beneficial applications in water and air
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Fig. 1. The deposition of nano-composite surfaces, formed from an Fe salt solution containing suspended INPs; top left nano-composites showing mixed cubic and spherical structures. Top right, focused ion beam (FIB) cross section demonstrating the deposited surface thickness The nano-composite is seen between the boxed region with a protective layer of Pt deposited above the surface and the substrate below. Bottom left, scanning transmission electron microscopy image of part of the nano-composite coating, illustrating the formation of cubic crystals over spherical nano-particle centres. Bottom right atomic force microscopy (AFM) data of a 5 m2 region.
filtration (industrial and domestic), and the potential for alternative uses in other areas of industry. By applying the coating method to a substrate of reticulated vitreous carbon (RVC) foam, a highly porous nano-composite suitable for both in situ and ex situ water filtration is created. The current work demonstrates how a reactive and nano-structured coating may be created throughout the filter and investigates the physiochemical benefits for the iron nanoparticles (INPs) of post synthesis vacuum heat treatment, which has previously been shown to improve the structure, and hence reactivity, of free INPs [25,26,28,29]. Data presented here demonstrates the arising filter material to be highly effective for the remediation of a range of volatile aromatic organic pollutants at rates and magnitudes competitive with well-established remediation agents; organoclay (OC) and granulated activated carbon (GAC).
the INP material. Approximate surface area was determined from the AFM data, using a surface area integration method [29], to be 3 ± 0.5 m2 g−1 of surface coating. X-ray photo-electron spectrometry (XPS) was used to confirm the oxidation state of the formed nano-composite surfaces, the presence and chemistry of surface contaminants (Fig. 2). The analysis confirmed the presence of both metallic iron and iron oxide in the surface analysis volume. With an approximate maximum sample depth of 6 nm (for the XPS analysis conditions) the recorded oxide signal is ascribed to the formation of a (<2 nm, Fig. 1 TEM image) surface oxide layer (<2–3 nm) on the deposited metallic
2. Results and discussion 2.1. Deposition of nano-composite iron surfaces Results are shown of a coating formed from an Fe salt solution containing suspended INPs (Fig. 1). Under empirically determined deposition variables nano-faceted surface coatings of metallic iron were successfully and repeatably deposited on vitreous carbon substrates, as shown in Fig. 1. The AFM and SEM data show the resulting nano-texture of the coatings to include both cubic and spherical structures. Using focused ion beam (FIB) cross-sectioning (Fig. 1), the surface coating was determined to between 50 and 100 nm in thickness; approximately equivalent to the average diameter of
Fig. 2. XPS of Fe 2p3/2 peaks of the nanocomposite surface formed from an Fe salt solution containing suspended INPs.
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iron coating. Spectrum curve fitting results of the recorded Fe2p photoelectron profile indicated an Fe/FeO oxide ratio of 0.1627 and Fe(II)/Fe(III) = 0.539. Consequently the inferred oxide stoichiometry is that of a near stoichiometric magnetite (Fe3 O4 ) grown on the metallic iron nano-coating, after formation of the composite This is in agreement to previously documented studies [26,28]. When considering the influencing parameters on the electrodeposition method a large number of variables were considered, examples images from each of the variables can be found in the appendix. Of all the parameters the deposition current was considered the most important for surface formation; a greater current induced a larger flux of ions towards the cathode [30], but also caused a more rapid evolution of hydrogen (from the generation of aqueous H+ ions). The production of H2 bubbles at the cathode prevented the metallic surface from binding to the substrate. Consequently the effect of current density was explored over a range of values and the resulting coverage of iron was closely examined by SEM and energy-dispersive X-ray spectroscopy (EDX). The most appropriate value was found to be approximately 0.09 A cm−2 . Electro-deposition time was considered another important factor; longer deposition times created a thicker coating layer; however, this rapidly became uneven and began to spall from the electrode surface. The optimum time period was identified through similar iterative testing but using SEM analysis and focused ion beam sectioning. Sonification of the electrolyte was performed to encourage the nano-particles to remain suspended in the solution and minimised sedimentation and aggregation; however, it simultaneously prevented the nano-particles from anchoring to the substrate and reduced crystallisation of the Fe2+ ions. Resultantly, a pulsed plating method was adopted whereby pulses of current were alternated with pulses of sonification, allowing the INPs to anchor to the substrate whilst concurrently preventing sedimentation. To obtain a thicker, uniform coating layer the method was further developed by using a reverse-plating technique. This technique adaptation reverses the polarity of the electrodes at regular intervals in order to disperse uneven accumulations of the nano-coating. Various time intervals were tested for electrode reversal and the results were studied via SEM to identify the best conditions. Whilst iron based systems were of primary interest the applicability of the coating process to other material systems was also investigated, specifically the Ag/AgN, TiO2 /AgN and Pt/PtCl mixed nano-particle:electrolyte systems were examined, Fig. 3. In the Ag/AgN system the deposition of similar nano-faceted surfaces coatings was readily achieved (Fig. 3). However due to the propensity for Ag to form small nano-crystals during deposition, silver dendrites were observed to form on top of the original (and somewhat larger) silver nano-particles. In this case the silver nanoparticles used were relatively large to illustrate that the particles act as nuclei for crystal formation from solution (dendritic structures shown in Fig. 3). For the TiO2 nano-particle:AgN salt system a highly structured nano-composite coating was readily formed and demonstrates the possibility of mixing different nano-particles types with different metals salts to form alternative composites. Due to the size of the particles, it is not necessary to use a permanently charged particle to be influenced by the electric field. TiO2 particles have been cemented together with silver salt deposited out of solution, and show smaller crystal grains growing on the surface of the particles. For the Pt/PtCl system results observed were very similar to that of the INP/Fe system. For a mixed phase system (containing PtNP/INP and corresponding salts) a uniform structure was also observed. However the deposition rate was greatly accelerated in comparison to the monometalic systems, with crystallisation of metallic Pt acting to rapidly cement the voids between particles. For the same coating periods as used for the INP/Fe system, coatings approximately 5 times thicker were formed which have
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Fig. 3. Nano-composites formed from difference deposition systems, with Ag nano-particles AgN salt (top left), Pt nano-particles PtCl3 salt (top right) TiO2 nanoparticles AgN salt (bottom left) Pt nano-particles FeCl3 salt (bottom right).
a significantly reduced surface area relative to other non-mixed metal systems. In all the systems studied the particle concentration was found to have a significant bearing on the nano-composites formed. This is evidenced by examining end members of the deposition concentrations, both suspensions of nano-particles and a pure salt solution. With each end member a different structure is formed (see Fig. 4). With a pure INP solution resulted in the generation of poorly distributed agglomerated masses of INP on the surface. This is ascribed to the particles acting as points of charge concentration with subsequent particles attracted to these point structures and resulting in accumulations of particle agglomerates tens to hundreds of microns in extent. Limited dissolution of the INP was considered to potentially provide a finite source of aqueous metal ions to cement the agglomerates as they formed. By comparison a salt only system deposited a smooth even metallic Fe layer with no
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Fig. 4. Nano-composites formed from an Fe salt solution containing suspended INPs illustrating the differences between a pure salt deposition (top left), a pure nanoparticle deposition (top right), and idealised proportions (50:50) of nanoparticles to salt in the solute (bottom left, bottom right).
nano-faceted surface structure. This was ascribed to the comparative lack of nucleation sites provided in the other systems by the nano-particles. Consequently nano-faceted coatings were only observed when the electrolyte contained both an aqueous salt and suspension of a significant quantity of nano-particles. It is proposed that the nano-particles act as nucleation sites for electrodeposition of the aqueous metallic salts [30]. By establishing low starting pH for the electrolyte, below that of the point of zero charge (PZC) of the nanoparticle surface, H+ ions functionalised the suspended particles, giving a net positive surface charge. It is proposed that the locally suspended INP are attracted to the cathode surface via an electrical charge effect. On contact with the surface the nano-particles subsequently act as seed-nuclei acting to lower the activation energy
for electrodeposition of the metal cations from the solution. This can be seen in the high resolution STEM images (Fig. 1), with nanoparticles shown at the substrate:layer interface. This mechanism is demonstrated to apply to both the metallic and non-metallic nano-particles examined here. It is considered that this mechanism is key to the deposition process with the significant number density of surface particles (estimated at 3 × 1013 cm2 ), preventing the electrodeposition of featureless metallic iron thin films on the conductive substrate. Although the surface charge resulting from the low pH allows the nano-particle to be influenced by the electric field the attraction is assumed to be relatively short range. This means that only nano-particles close to the surface of the substrate can actually be attracted, fixed and therefore can act as nucleation sites for the
Fig. 5. Nano-composites formed from an Fe salt solution containing suspended INPs and illustrates the surface of the nano-composite at the top of the formed filter material (right) and middle of the filter material (left).
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Fig. 6. The percentage of organic removed normalised by surface area of reactive material (INC 12 m2 g−1 , GAC 65 m2 g−1 , OC 825 m2 g−1 , determined by BET analysis).
deposition. To this end the pulsed sonification allows the particles to remain suspended throughout the fluid in an even concentration, and not depleted in the bonding region due to the deposition of nano-particles on the surface. Furthermore sonification prevents the aggregation and clumping of nano-particles within system, without the use of surfactants or other chemicals that would alter the surface chemistry of the nano-particle and would normally be used to prevent aggregation. Sonification also has the added benefit of desorbing H2 bubbles from the surface of the cathode. Pulsed sonification was found to display the best results as interval pulsing was sufficient to prevent nano-particle aggregation, without resulting in degradation of the surface coating, as continual sonification was observed to remove nano-particles loosely bound to the substrate surface. The variable found to be key to the uniformity and repeatability of the coating process was the applied current density. The current density had to be high enough to draw the ions to the surface; however, at low pH H+ ions are also drawn to the cathode and result in the generation of H2 gas. This evolution of gas
prevents the formation of a stable, regular surface. Consequently at high current densities (>1 A cm−2 ) the formed surfaces are cracked and inconsistent with bubble like structures observed in the coatings. Even at low current densities, polarity reversal of the electrodes was necessary to mitigate the rapid build-up of materials at points of charge concentration. This method was found to provide repeatably uniform surface coatings (thickness and morphology).
2.2. Water remediation The refined electrodeposition process creates a nano-structured layer on the surface of the carbon throughout the porous structure (Fig. 5). The coating consists primarily of quasi spherical features, approximately 20–50 nm in size, created by the deposition of INPs cemented to the surface by the dissociated iron from FeCl3 . These features are interspersed by faceted metallic crystals, resulting
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Fig. 7. A schematic diagram of experimental setup.
Fig. 8. A schematic diagram of experimental setup for filter production.
from the INPs acting as nucleation sites for heterogeneous crystal growth. The study compared the performance of OC, GAC and the iron nano-composite (INC) in remediating BTEX organic compounds (benzene, toluene, ethyl-benzene and m-, p-, o-xylene). The results from ChemTest Ltd. demonstrated that the INC exhibited BTEX uptake at rates and magnitudes very closely comparable with OC and GAC. However, once this data is normalised by the surface area of the reactive material, it becomes clear that the INC significantly outperforms the comparator materials (Fig. 6). Based on other INP studies [7] the mechanism involved for BTEX removal onto the INC developed is considered to be adsorption followed by degradation caused by the organic-iron coordination weakening of the benzene ring. This is compared to OC and GAC which remove organics via adsorption alone [35,36]. The result of this investigation implies that the INC is more reactive than traditional agents, having beneficial implications for both cost and resources required for remediation. Further work will explore the application of this coating method to different sectors of water remediation including heavy metals, radionuclides and complex organic molecules.
3. Conclusions We present here a novel technique for the anchoring of nanoparticles on to a conductive substrate to form a nano-composite. The technique has been demonstrated as highly effective at binding both metallic and non-metallic nano-particles with a variety of aqueous metallic salts systems, including NP/salt systems of: Fe/FeCl3 , Ag/AgN, Pt/PtCl3 and TiO2 /FeCl3 . The nano-composites formed are observed to have a uniform, nano-faceted surface covering centimetre sized areas of the substrate. The key conclusions here presented are that a variety of metallic and non-metallic nano-particles have been deposited on a conductive carbon glass substrate including NP/salt systems of: Fe/FeCl3 , Ag/AgN, Pt/PtCl3 and TiO2 /FeCl3 . The formed nano-faceted surfaces show highly enhanced reactive surface areas (3 ± 0.5 m2 g−1 of surface coating). Current density (<0.1 A cm−2 ) and particle concentration (50:50 salt:NP) used for deposition were determined as the most important factors for successful deposition. However, sonification and reverse plating of the substrate offered further improvements. The substrates can be re-cycled by applying a reverse current, which removes the surface coating. The deposition technique is low technology, low energy and does not
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Fig. 9. Secondary electron micrograph images demonstrating the effect of the deposition variables on the deposited surface, formed from an INP FeCl3 system on a reticulated foam. The subfigures explore: deposition time (a) 10 min, (b) 25 s; FeCl3 :INPs ratio, (c) 1:3, (d) 1:1; substrate porosity, (e) 20 ppi, (f) 30 ppi; electrodeposition current, (g) 0.1A, (h) 0.2A, (i) 0.2A, (j) 0.3A; pulsed current (outside surface of foam), (k) no pulsed plating, (l) pulsed plating; pulsed current (centre of foam), (m) no pulsed plating, (n) pulsed plating; reverse current pulses, (o) forward current pulses, (p) forward and reverse current pulses; pulse length (q) 60 s forward, 10 s reverse (r) 20 s forward, 10 s reverse.
require the use of expensive precursors, and is considered applicable to a large range of nano-particle types. A demonstration of the INP/FeCl3 nanocomposites as a water filter is presented, which out performs current market leading materials. 4. Experimental 4.1. Nano-particles synthesis Nano-structures were manufactured in the present work using nano-particles of metallic iron (INP), titanium dioxide (TiO2 ), platinum (Pt) and silver (Ag). The materials were selected for their well-documented reactivity and respective capabilities for pollutant remediation. They represent the nano-materials most commonly used on a commercial basis and may be readily synthesised or purchased. For example, INPs are used for environmental remediation purposes as they are: (i) highly reactive, (ii) low cost,
(iii) available in kg quantities, (iv) environmentally compatible, and (v) form no hazardous by-products. Bi-metallic iron–nickel nano-particles, (BNP), were synthesised following an adaptation of the method first described by Wang and Zhang [31], using sodium borohydride to reduce ferrous Fe to a metallic state. Briefly, 7.65 g of FeSO4 7H2 O was dissolved in 50 ml of Milli-Q water (18.2 M cm) and then a 4 M NaOH solution was used to adjust the pH to the range 6.2–7.0. The salts were reduced to metallic nano-particles by the addition of 3.0 g of NaBH4 . The product was isolated through centrifugation and then sequentially washed with water, ethanol and acetone (20 ml of each). The nano-particles were dried in a desiccator under low vacuum (10−2 mbar) for 48 h and then stored in a nitrogen-filled glovebox (Saffron Scientific) until required. Platinum nano-particles were formed via a modified [31] method, in which, 0.017 g PtCl4 (IV) (Sigma Aldrich) was dissolved
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Fig. 9. (Continued )
in 100 ml of Milli-Q water. The Pt salt solution was added dropwise to a 0.002 mol ml solution of NaBH4 solution under constant agitation. The arising nano-slurry was centrifuged and the supernatant decanted. The residual solid was washed in Milli-Q water, before further centrifugation and the final residual solid was stored with in a vacuum desiccator (>10−2 mbar). Silver nano-particles were also formed in the same way to the Pt nanoparticles using the [31] method; however, 0.017 g AgN (IV) (Sigma Aldrich) dissolved in 100 ml of Milli-Q water as the starting salt solution. TiO2 nano-particles were sourced commercially (Degussa P25). The metallic salts used (FeCl3 , AgN, PtCl4 ) were all of analytical grade (Fisher Chemicals). 4.2. Material selection In the present study INP were selected for the most significant analytical characterisation due to its widespread use compared to
the aforementioned nanomaterials [32–34]. Furthermore BNP was selected to use nickel as a tracer, for locating where the particles are deposited and allows them to be distinguished from Fe deposited from an aqueous ionic state. As there is no nickel present within the aqueous solution, the detection of nickel clusters by spectrometry techniques indicates material deposited as nano-particles, and not from the aqueous salt solution, which forms the cementing matrix. 4.3. Deposition method A pre-fabricated vitreous carbon block was used as a conductive substrate (Sigradra, HTW Hochtemperatur Werkstoffe GmbH) onto which the nano-particles were anchored via electrodeposition. The carbon glass acted as the cathode, and a stainless steel beaker was used as the anode and containing vessel for the electrolyte solution (Fig. 7). The electrolyte solution (approximately pH 3 throughout) contained Milli-Q water with appropriate concentrations and ratios
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Fig. 9. (Continued )
of metallic salt and nano-particles. The steel beaker was sonicated in order to keep the nano-particles in suspension and with minimal aggregation. The optimum concentrations were identified after an iterative programme of empirical testing. For deposition on to a vitreous carbon block, optimal deposition was found when a concentration of 5 g L−1 nanoparticle (i.e. INP, TiO2 , Pt, Ag) with 5 g L−1 of appropriate salt (i.e. FeCl3 , AgN, PtCl4 ) was used. 4.4. Filter formation In order to produce a water filter the above deposition method was modified. The substrate discs, 9 mm thick and 47 mm in diameter, of reticulated vitreous carbon (RVC) foam (ERG Aerospace, U.S.A.) with a porosity of 45PPI, were washed ultrasonically in acetone and Milli-Q water as a preparatory cleaning step. The RVC foam discs were maintained in contact with a stainless steel plate forming the cathode. A stainless steel beaker acted as the anode and container for the electrolyte solution, which was a combination
of 0.4 g FeCl3 (99.9% Sigma) and 1 g INPs (NanoIron Ltd., Czech Republic) in 200 ml deionised water, solution pH was not modified and was measured as pH 4. The beaker was held within a sonic bath to ensure the suspension of the INPs (Fig. 8). Before addition of INPs, the electrolyte solution was purged of oxygen using argon to limit the aqueous oxidation of the INPs. The RVC disc was sonicated in the electrolyte before beginning the deposition to allow the INPs to penetrate the foam. A 9 min electrodeposition cycle was performed comprising repeating sets of an initial 40 s period consisting of alternate 5 s bursts of sonication and then current pulses, followed by a 20 s period of alternating 5 s bursts of sonication and reverse current pulses. The foam was then turned over and the cycle repeated. The samples were then rinsed with acetone to remove any excess INPs and placed immediately in a vacuum desiccator to dry. Control samples were formed using electrolyte solutions void of nanoparticles (0.0086 mol FeCl3 ). Resulting nano-structures were analysed using scanning electron microscopy and also using atomic force microscopy.
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A series of samples were vacuum annealed at 600 ◦ C for 24 h at 10−5 mbar. X-ray photoelectron spectroscopy was performed both before and after the vacuum annealing to investigate the change in surface physiochemistry. Organic contaminant sorption tests were performed by ChemTest Ltd. to compare the remedial abilities of the sorbents granular activated carbon (GAC), organo-clay (OC) and the vacuum annealed nano-composite. A groundwater sample was abstracted from a contaminated site in Portsmouth, UK, containing benzene, toluene, ethyl-benzene, and m-, p-, o-xylene (the BTEX group). Kinetic experiments were conducted using continuously stirred 1000 ml glass sealed reactors at a suspension density of 0.4 g L−1 . Samples were taken at time points (t) = 5, 10, 30, 90, 360, 1440 min. Solution chemistries were determined by headspace gas chromatography mass spectrometry for volatile organic compounds. Acknowledgements The authors thank Chris Jones for TEM preparation and to Julia Greenwood for laboratory assistance in this project. Appendix A. Fig. 9. References [1] J.W. MacFarlane, H. Jenkinson, T. Scott, Applied Catalysis B: Environmental 106 (2011) 181–185. [2] M. Redetic, Journal of Materials Science 48 (1) (2011) 95–107. [3] A. Ghauch, A. Tuqan, H.A. Assi, Environmental Pollution 157 (5) (2009) 1626–1635. [4] Z. Fang, J. Chen, X. Qiu, X. Qiu, W. Cheng, L. Zhu, Desalination 268 (13) (2011) 60–67. [5] H. Tian, J. Li, Z. Mu, L. Li, Z. Hao, Separation and Purification Technology 66 (1) (2009) 84–89. [6] S. Klimkova, M. Cernik, L. Lacinova, J. Filip, D. Jancik, R. Zboril, Chemosphere 82 (8) (2011) 1178–1184. [7] R.A. Crane, T.B. Scott, Journal of Hazardous Materials 211–212 (0) (2012) 112–125. [8] P. Calcagnile, D. Fragouli, I.S. Bayer, G.C. Anyfantis, L. Martiradonna, P.D. Cozzoli, R. Cingolani, A. Athanassiou, ACS Nano 6 (6) (2012) 5413–5419.
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