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Electrochemistry in nanoparticle science
Current Opinion in Colloid & Interface Science 7 Ž2002. 186᎐192
Electrochemistry in nanoparticle science D. Jason Riley U School of Chemistry, Uni¨ ersity of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
Abstract Electrochemistry is a key technology in nanoparticle science. For example electroplating offers novel routes to nanosized particles via arrested and templated electrodeposition. As the science underlying the preparation and assembly of nanoparticles matures methods of exploiting the novel properties of these new functional materials are being scrutinised. Electrochemistry is a suitable method for coupling particle activity to external circuitry. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Nanoparticles; Q-dots; Self-assembly; Electrochemistry; Heterojunctions
1. Introduction Interest in the formation, characterisation and applications of nanoparticles continues unabated. The plot in Fig. 1 shows how the number of publications cited on the ISI database that contain the word nanoparticleŽs. in the title, representing only a fraction of the total papers published in the field, has increased since the middle of the 1980s. Initial studies of nanoparticles focused on their unique properties that stem from their high surface area to volume ratio and the spatial confinement of the charge carriers w1,2x. Recently, this work has been complemented by studies of functional materials formed by the controlled assembly of nanoparticles w3,4x. This article will describe the contributions of electrochemistry to recent advances in nanoparticle science in fields as diverse as optoelectronics and biological sensors. Electrochemistry is an enabling technology across the whole spectrum of nanoparticle research, from particle preparation to sensor applications and from bandgap determination to solar cell operation. After a look at how electrochemistry may
U
Tel.: q44-117-928-7668 fax: q44-117-925-0612. E-mail address: [email protected] ŽD. Jason Riley..
be employed in nanoparticle preparation the article will focus on how nanoparticles formed using colloid chemistry can be addressed electrochemically. Throughout the review the focus will be on how electrochemistry can aid studies of nanoparticles.
2. Electrodepsoition as a route to nanoparticles In colloid science nano-sized particles are often prepared from supersaturated solutions containing organic surfactants. The primary role of the organic ligands is to passivate the surface and suspend growth. This technique of nanoparticle preparation is termed arrested precipitation. Analogous methods for the formation of nanoparticles on conducting substrates have been developed in electrochemistry. It has been demonstrated that the addition of surface capping agents can lead to the deposition of nanoparticles during electroplating w5x. The additives serve to arrest particle growth and the strength of the absorption correlates to the particle size. A more general method to suspend particle expansion is the regulation of the electrodeposition parameters; current or voltage w6,7x. A two step procedure is employed, a high overpotential is applied for a short period to nucleate metal particles at the surface. Then to slowly grow the
1359-0294r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 0 2 9 4 Ž 0 2 . 0 0 0 4 7 - X
D.J. Riley r Current Opinion in Colloid & Interface Science 7 (2002) 186᎐192
Fig. 1. The number of publications on the ISI database each year that contain the word nanoparticleŽs. in the title.
particles to the required final dimension a low overpotential pulse is applied. The low overpotential promotes a modest variation in particle size, typically less than 7%, as it both prevents coupling of the diffusion layers of adjacent particles and results in a particle growth rate that decreases with increasing particle dimension. The shape of the particles produced by arrested electrodeposition depends on the substrate employed and the applied overpotential. Metal; including gold, silver and nickel w6,7x; and conducting polymer nanoparticles w8x with spherical geometry have been prepared on basal graphite planes. Also on graphite spherical nanoparticles are formed when a high overpotential is employed w9x. However, by careful selection of experimental conditions it is possible to form nanowires on graphite electrodes w10,11x. This has been achieved by selecting potentials that restrict electrodeposition to step edges. Palladium nanowires, as narrow as 55 nm in diameter and hundreds of m in length, prepared using this method, have been transferred to a polymer matrix and used as hydrogen sensors, the resistivity of the wires decreasing on exposure to H 2 w11x. Material growth using porous templates to control size and shape would appear to be a universal route to nanoparticles. However, for the majority of deposition routes, e.g. precipitation and chemical vapour deposition, problems arise due to the non-uniform distribution of product and the plugging of pores. These issues are circumvented in templated electrodeposition, see Fig. 2, as material growth can only take place at the pore base w12᎐14x. Template electrodes have been formed on a range of materials
187
including track-etched mica w15x and porous alumina membranes w16x. Templated electrodeposition has been used to prepare nanowires of a range of materials. For example, the methodology was employed to prepare suspensions of fluorescently labelled nickel wires, of nanometer radius and micron length, that allowed the direct visualisation of the effect of magnetic field on particle alignment w17x. In an ingenious development of the template synthesis technique patterned nanowires have been grown by periodically moving the working electrode between a solution containing gold ions and a solution of silver ions w18x. The contrast in reflectivity of the gold and silver sections yields wires that have been described as nano-barcodes. It is envisaged that in the future such nanorods will be used in bioassays, the barcode element allowing the chemical history of the particle to be traced. In an interesting variation of the above, templated electrodeposition has been employed to prepare high surface area material containing nanosized cavities w19,20x. Arrays of spherical polystyrene nanoparticles were crystallised on to a gold substrate from a colloidal sol. On electrodeposition of a metal on the resultant electrode a metal-polystyrene composite was formed. The polystyrene particles were then dissolved to yield a metal layer with nanosized voids. These cavities supported localised surface plasmons and resulted in sharp spectral features in the film reflectivity. The reflectivity of the film could be tuned by varying the size of the polystyrene particle employed in the deposition process w20x.
3. Charge transfer to and from Q-dots Particles that, as a result of the spatial confinement of charge carriers, display properties that differ from those of the chemically identical bulk material are termed Q-dots. The ability to tune the property of a Q-dot via control of its size has led many scientists to postulate that such particles will be the basis of the electronic devices of the future. To assess this assertion it is necessary to investigate the effects and kinetics of charge transfer at Q-dots. The difficulty of wiring the particles is an obvious obstacle to such studies. Whilst STM has allowed single particles to be studied w21᎐23x, a host of electrochemical techniques have been employed to investigate the electronic properties of arrays of nanoparticles. In the majority of such experiments the electrode and the electrolyte act as the ‘wires’ that couple the particle to the circuit. In electrochemical studies of electrolyte containing metal nanoparticles the size and dispersivity of the particles are key issues w24᎐26x. A metal typically has
188
D.J. Riley r Current Opinion in Colloid & Interface Science 7 (2002) 186᎐192
Fig. 2. Schematic illustrating the steps involved in the templated electrodeposition of nanowires.
a capacitance of tens of microfarads per cm2 and charging of the bulk material is continuous. However, for a metal Q-dot, that is stabilised by a dielectric layer of organic surfactant and is suspended in a solution of low dielectric constant, the capacitance, C, can be less than an attofarad. The energy required to add a single electron to such a particle, e r C, is greater than k B T r e, i.e. the charging of the particle is quantised. Therefore, on performing a potential scan on a solution containing metal Q-dots all of similar capacitance, i.e. a monodisperse size distribution, the discontinuity in particle charging leads to current peaks. Each peak corresponds to a single electron transfer event between the electrode and those metal particles near the surface. The separation of the peaks depends on C and hence, the particle size. As the rate of charge transfer is limited by transport of Q-dots to the electrode surface analysis of the current allows the diffusion coefficient of the particles to be calculated w25x. It is more usual to perform electrochemical studies
of charge transfer in Q-dot systems using self-assembled films of the nanoparticles. It has proved possible to deposit multilayers of suitably surface functionalised metal nanoparticles onto conducting substrates via ion bridging w27x. The resultant modified electrodes show evidence of quantised charging. Charge transport at such modified electrodes requires electrons to tunnel across the organic layers that separate the nanoparticles. Studies of the current time transient for film charging allows the rate of electron hopping to be determined. It is apparent that the rate of electron transfer between a nanoparticle and the substrate will be influenced by the organic moiety that separates the two. An organic chain with a redox active group offers a particularly elegant illustration of this point w28x. Gold nanoparticles were deposited on a gold electrode using a dithiol link, consisting of 20 methylene groups with a bipyridinium at the centre. An STM tip was employed to investigate ease of electron tunnelling between the nanoparticle and the substrate as the oxidation state of the
D.J. Riley r Current Opinion in Colloid & Interface Science 7 (2002) 186᎐192
bipyridinium group was electrochemically altered. The lowest resistance was obtained when the bipyridinium moiety was able to act as an electron shuttle. For metal Q-dots deconvolution of the current flowing at the electrode surface, the short circuit current, and that at the nanoparticle surface can prove a problem. This issue is easily avoided for electrodes modified with semiconductor Q-dots where photoelectrochemical studies yield signals that originate only from those nanoparticles that absorb the incident light. Simple photocurrent spectroscopy allows the absorption spectrum of the particles at a modified electrode interface to be measured, yielding information on the size of the Q-dots at the surface w29x. Using intensity modulated photocurrent spectroscopy, a technique in which the magnitude and phase of the photocurrent is measured relative to a sinusoidally modulated incident light intensity, the kinetics of charge transfer at semiconductor nanoparticle solution interfaces has been studied w30᎐34x. The influence of the length of the spacer molecule on the rate of electron transfer from CdSe Q-dots to a gold substrate has been studied w31x. The particles were linked to the electrode surface using a series of rigid disulfide molecules of varying length. The rates of electron transfer between the conduction band of the nanoparticle and the gold and the gold and trap states of the CdSe were both found to have an exponential dependence on the length of the spacer. The ability to control electron injection into the semiconductor Qdots allows the influence of charge on the optical properties of the particles to be investigated. Using CdS modified electrodes the bleaching of particle absorbance by electron injection has been studied w33,35x. The results obtained indicate that the change in absorbance is due to a Moss᎐Burstein shift in the bandedge as bleaching is only observed when potentials are reached for which it is thermodynamically favourable to inject electrons from the substrate to the Q-dot conduction band. Studies of semiconductor Q-dots may also be performed in the dark. In a recent paper w36x the electrochemistry of CdS Q-dots suspended in N, N⬘-dimethylformamide was reported. From the onsets of the anodic and cathodic currents the energies of the conduction band and valence band of the nanoparticle can be determined. The separation between the semiconductor bands as measured electrochemically matched the optical bandgap of the Q-dots. In the work described above the electrode acts as one of the contacts to the Q-dot. This need not be the case and it is possible to couple a nanoparticle to a circuit via electrolyte only. This has been achieved w37x using Scanning Electrochemical Microscopy ŽSECM.. A schematic of the technique as applied to nanoparticle layers is shown in Fig. 3. It is apparent
189
Fig. 3. Schematic showing how SECM using methyl viologen as a probe may be employed to investigate the properties of gold nanoparticles deposited on an insulating substrate. Note that the components of the drawing are not to scale, typically the gold nanoparticles are less than 10 nm diameter and the ultramicroelectrode is 25 m in diameter w37x.
that the current flowing due to the methyl viologen dication reduction at the ultramicroelectrode is dependent on the rate of the reaction, Au
MVqq Hq ª MV 2qq H 2 , at the gold nanoparticles.
4. Assemblies of nanoparticles The fabrication of novel commercial devices based on assemblies of nanoparticles is the goal of much nanoparticle research. As discussed above electrochemistry offers an easy route of coupling nanoparticle activity to an external circuit and is therefore often key to the realisation of a working device. A major advantage of nanoparticle assemblies over conventional materials is the high surface area to volume ratio. For example, a nanocrystalline TiO 2 film of 10 m thickness may have an internal surface area approximately 1000 times greater than the geometric area. It is the high surface area that, when coated in dye, leads to the high absorbance and hence, efficient light harvesting of dye sensitized solar cells, a technology recently reviewed in this journal w38x. This idea of enhancing the optical absorption of a film by increasing the internal surface area has recently been exploited in a new generation of electrochromic displays w39᎐41x. It has been demonstrated that by attaching suitable redox active chromophores to transparent nanostructured TiO 2 and SnO 2 films
190
D.J. Riley r Current Opinion in Colloid & Interface Science 7 (2002) 186᎐192
Fig. 4. Schematic illustrating how a DNA sensor can be built using nanoparticle assembly. A photocurrent from the assembly indicates the presence of the analyte DNA. CdS1 and CdS2 are nanoparticles coated with DNA-1 and DNA-2, respectively w48x.
supported on conducting glass a display device with excellent performance characteristics can be fabricated. It is of note that the mechanism of chromophore oxidation at the TiO 2 surface is not yet fully understood. The high surface area of nanoparticle modified electrodes is also attractive in catalysis w42,43x and electrochemical sensor w44,45x applications. An example of the latter is the electroanalysis of proteins electrostatically attached to metal oxide surfaces w46,47x. The nanoparticulate surface not only enhances current due to the high surface area but the scale of surface roughness removes the requirement for a solution phase species to mediate electron transfer to the active redox site of the protein. A more difficult means of exploiting nanoparticles, that in the long term will probably prove more rewarding, is the development of devices that require a high degree of architectural integrity. Work in this area is in its infancy. So far, the alteration of surface groups to enhance nanoparticle functionality and the fabrication of semiconductor heterojunctions has been considered. A particularly illustrative example of the former is the DNA sensor w48x shown in Fig. 4. It is
apparent that the photoelectrochemical signal from the modified electrode depends on the amount of analyte present in solution. Enhancement of the signal in the sensor was attained by adding wRuŽNH 3 . 6 x 3q, it is postulated that the ions bind electrostatically to the DNA and mediate the transfer of photogenerated charge carriers. Charge transfer in sols of composite semiconductor nanoparticles has been extensively studied w49,50x, however, investigations of assemblies of mixed semiconductor nanoparticles are limited. A method of depositing CdS nanoparticles on nanocrystalline TiO 2 electrodes that permits control of particle size has recently been reported w51x. It was shown that photogenerated charge carriers are transferred from the CdS to the TiO 2 substrate, offering the possibility of building a sensitized solar cell without the need for an organic dye to absorb the light.
5. Conclusions For the past decade there has been interest in
D.J. Riley r Current Opinion in Colloid & Interface Science 7 (2002) 186᎐192
isolated nanosized particles as they possess novel physical and chemical properties. The recent realisation that the controlled assembly of nanoparticles can further enhance material attributes has led to increased activity in the field. The exploitation of Q-dots in devices requires that methods of transducing their activity are developed. Above we have demonstrated that electrochemistry offers an excellent means of contacting nanoparticles; allowing the influences of charge and potential on particle properties to be investigated and the development of sensor, electrochromic and solar cell devices in which electrochemistry is used to address nanoparticle arrays. In the future it is likely that electrochemistry will remain the method of choice for investigating the electronic properties of new materials. Whether, electrochemistry will remain a dominant technology as devices reach commercialisation will depend on whether the issues surrounding the use of liquid electrolytes can be addressed. Thus far the range of electrochemical methods employed in nanoscience has been limited. If electrochemistry is to remain at the forefront of this field advanced techniques must be employed in order to address some of the phenomena recently observed in Q-dot assemblies. For example, to study the ability of redox ions to mediate charge transfer from nanoparticles methods of decoupling the ion and nanoparticle electrochemistry will be required. Acknowledgements We are grateful to the Royal Society, Nuffield Foundation and EPSRC for their support of our research in the electrochemistry of nanoparticulate systems. References w1x Trindade T, O’Brien P, Pickett NL. Nanocrystalline semiconductors: Synthesis, properties, and perspectives. Chem Mat 2001;13:3843᎐3858. w2x Zhang JZ. Interfacial charge carrier dynamics of colloidal semiconductor nanoparticles. J Phys Chem B 2000;104: 7239᎐7253. w3x Fendler JH. Chemical self-assembly for electronic applications. Chem Mat 2001;13:3196᎐3210. w4x Shipway AN, Willner I, Nanoparticles as structural and functional units in surface- confined architectures. Chem Commun 2001; 2035᎐2045. w5x Mastai Y, Gal D, Hodes G. Nanocrystal-size control of electrodeposited nanocrystalline semiconductor films by surface capping. J Electrochem Soc 2000;147:1435᎐1439. w6x Liu H, Favier F, Ng K, Zach MP, Penner RM. Size-selective electrodeposition of meso-scale metal particles: a general method. Electrochim Acta 2001;47:671᎐677. w7x Penner RM. Mesoscopic metal particles and wires by electrodeposition. J Phys Chem B 2002;106:3339᎐3353. w8x Tang ZY, Liu SQ, Wang ZX, Dong SJ, Wang EK. Electrochemical synthesis of polyaniline nanoparticles. Electrochem Commun 2000;2:32᎐35.
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w9x Zach MP, Penner RM. Nanocrystalline nickel nanoparticles. Adv Mater 2000;12:878᎐883. w10x Zach MP, Ng KH, Penner RM. Molybdenum nanowires by electrodeposition. Science 2000;290:2120᎐2123. w11x Favier F, Walter EC, Zach MP, Benter T, Penner RM. Hydrogen sensors and switches from electrodeposited palladium mesowire arrays. Science 2001;293:2227᎐2231. w12x Sun L, Searson PC, Chien CL. Finite-size effects in nickel nanowire arrays. Phys Rev B 2000;61:R6463᎐R6466. w13x Sapp SA, Mitchell DT, Martin CR. Using template-synthesized micro- and nanowires as building blocks for self-assembly of supramolecular architectures. Chem Mat 1999;11: 1183᎐1185. w14x Davydov DN, Haruyama J, Routkevitch D et al. Nonlithographic nanowire-array tunnel device: Fabrication, zero-bias anomalies, and Coulomb blockade. Phys Rev B 1998;57: 13550᎐13553. w15x Sun L, Chien CL, Searson PC. Fabrication of nanoporous single crystal mica templates for electrochemical deposition of nanowire arrays. J Mater Sci 2000;35:1097᎐1103. w16x Masuda H, Ohya M, Asoh H, Nishio K. Photonic band gap in naturally occurring ordered anodic porous alumina. Jpn J Appl Phys Part 2 2001;40:L1217᎐L1219. w17x Tanase M, Bauer LA, Hultgren A et al. Magnetic alignment of fluorescent nanowires. Nano Lett 2001;1:155᎐158. w18x Nicewarner-Pena SR, Freeman RG, Reiss BD et al. Submicrometer metallic barcodes. Science 2001;294:137᎐141. w19x Bartlett PN, Birkin PR, Ghanem MA, Toh CS. Electrochemical syntheses of highly ordered macroporous conducting polymers grown around self-assembled colloidal templates. J Mater Chem 2001;11:849᎐853. w20x Coyle S, Netti MC, Baumberg JJ, Ghanem MA, Birkin PR, Bartlett PN, Whittaker DM, Confined plasmons in metallic nanocavities. Phys Rev Lett 2001, 8717:art. no.-176801. w21x Alperson B, Rubinstein I, Hodes G, Identification of surface states on individual CdSe quantum, dots by room-temperature conductance spectroscopy. Phys Rev B 2001, 6308:art. no.-081303. w22x Bakkers E, Hens Z, Zunger A et al. Shell-tunneling Spectroscopy of the single-particle energy levels of insulating quantum dots. Nano Lett 2001;1:551᎐556. w23x Bakkers E, Vanmaekelbergh D. Resonant electron tunneling through semiconducting nanocrystals in a symmetrical and an asymmetrical junction. Phys Rev B 2000;62:R7743᎐R7746. w24x Zamborini FP, Gross SM, Murray RW. Synthesis, characterization, reactivity, and electrochemistry of palladium monolayer protected clusters. Langmuir 2001;17:481᎐488. w25x Chen SW, Sommers JM. Alkanethiolate-protected copper nanoparticles: Spectroscopy, electrochemistry, and solid-state morphological evolution. J Phys Chem B 2001;105:8816᎐8820. w26x Chen SW, Templeton AC, Murray RW. Monolayer-protected cluster growth dynamics. Langmuir 2000;16:3543᎐3548. w27x Zamborini FP, Hicks JF, Murray RW. Quantized double layer charging of nanoparticle films assembled using carboxylaterŽCu2q or Zn2q .rcarboxylate bridges. J Am Chem Soc 2000;122:4514᎐4515. w28x Gittins DI, Bethell D, Schiffrin DJ, Nichols RJ. A nanometre-scale electronic switch consisting of a metal cluster and redox-addressable groups. Nature 2000;408:67᎐69. w29x Torimoto T, Tsumura N, Nakamura H et al. Photoelectrochemical properties of size-quantized semiconductor photoelectrodes prepared by two-dimensional cross-linking of monodisperse CdS nanoparticles. Electrochim Acta 2000;45:3269᎐3276. w30x Bakkers E, Roest AL, Marsman AW et al. Characterization of photoinduced electron tunneling in goldrSAMrQ-CdSe
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w31x w32x w33x
w34x w35x
w36x w37x w38x w39x w40x
D.J. Riley r Current Opinion in Colloid & Interface Science 7 (2002) 186᎐192 systems by time-resolved photoelectrochemistry. J Phys Chem B 2000;104:7266᎐7272. Bakkers E, Marsman AW, Jenneskens LW, Vanmaekelbergh D. Distance-dependent electron transfer in AurspacerrQCdSe assemblies. Angew Chem-Int Edit 2000;39:2297᎐2299. Bakkers E, Kelly JJ, Vanmaekelbergh D. Time resolved photoelectrochemistry with size-quantized PbS adsorbed on gold. J Electroanal Chem 2000;482:48᎐55. Hickey SG, Riley DJ, Tull EJ. Photoelectrochemical studies of CdS nanoparticle modified electrodes: Absorption and photocurrent investigations. J Phys Chem B 2000;104: 7623᎐7626. Hickey SG, Riley DJ. Intensity modulated photocurrent spectroscopy studies of CdS nanoparticle modified electrodes. Electrochim Acta 2000;45:3277᎐3282. Riley DJ, Tull EJ. Potential modulated absorbance spectroscopy: an investigation of the potential distribution at a CdS nanoparticle modified electrode. J Electroanal Chem 2001;504:45᎐51. Haram SK, Quinn BM, Bard AJ. Electrochemistry of CdS nanoparticles: A correlation between optical and electrochemical band gaps. J Am Chem Soc 2001;123:8860᎐8861. Zhang J, Lahtinen RM, Kontturi K, Unwin PR, Schiffrin DJ, Electron transfer reactions at gold nanoparticles. Chem Commun 2001; 1818᎐1819. Gratzel M. Mesoporous oxide junctions and nanostructured solar cells. Curr Opin Colloid Interface Sci 1999;4:314᎐321. Sotomayor J, Will G, Fitzmaurice D. Photoelectrochromic heterosupramolecular assemblies. J Mater Chem 2000;10: 685᎐692. Merrins A, Kleverlaan C, Will G, Rao SN, Scandola F, Fitzmaurice D. Time-resolved optical spectroscopy of heterosupramolecular assemblies based on nanostructured TiO 2 films modified by chemisorption of covalently linked ruthenium and viologen complex components. J Phys Chem B 2001;105:2998᎐3004.
w41x Gratzel M. Materials science-Ultrafast colour displays. Nature 2001;409:575᎐576. w42x Zhong CJ, Maye MM. Core-shell assembled nanoparticles as catalysts. Adv Mater 2001;13:1507᎐1511. w43x Hayden BE, Malevich DV, Pletcher D. Platinum catalysed nanoporous titanium dioxide electrodes in H 2 SO4 solutions. Electrochem Commun 2001;3:395᎐399. w44x Israel LB, Kariuki NN, Han L, Maye MM, Luo J, Zhong CJ. Electroactivity Of Cu2q at a thin film assembly of gold nanoparticles linked by 11-mercaptoundecanoic acid. J Electroanal Chem 2001;517:69᎐76. w45x Labande A, Ruiz J, Astruc D. Supramolecular gold nanoparticles for the redox recognition of oxoanions: Syntheses, titrations, stereoelectronic effects, and selectivity. J Am Chem Soc 2002;124:1782᎐1789. w46x Topoglidis E, Cass AEG, O’Regan B, Durrant JR. Immobilisation and bioelectrochemistry of proteins on nanoporous TiO 2 and ZnO films. J Electroanal Chem 2001;517:20᎐27. w47x Topoglidis E, Campbell CJ, Cass AEG, Durrant JR. Factors that affect protein adsorption on nanostructured titania films. A novel spectroelectrochemical application to sensing. Langmuir 2001;17:7899᎐7906. w48x Willner I, Patolsky F, Wasserman J. Photoelectrochemistry with controlled DNA-cross-linked CdS nanoparticle arrays. Angew Chem-Int Edit 2001;40:1861᎐1864. w49x Wood A, Giersig M, Mulvaney P. Fermi level equilibration in quantum dot-metal nanojunctions. J Phys Chem B 2001;105:8810᎐8815. w50x Sant PA, Kamat PV. Interparticle electron transfer between size-quantized CdS and TiO 2 semiconductor nanoclusters. Phys Chem Chem Phys 2002;4:198᎐203. w51x Peter LM, Riley DJ, Tull EJ, Wijayantha KGU, Photosensitization of nanocrystalline TiO 2 by self-assembled layers of CdS Q-dots. Chem. Comm. in press.
Electrochemistry in nanoparticle science D. Jason Riley U School of Chemistry, Uni¨ ersity of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
Abstract Electrochemistry is a key technology in nanoparticle science. For example electroplating offers novel routes to nanosized particles via arrested and templated electrodeposition. As the science underlying the preparation and assembly of nanoparticles matures methods of exploiting the novel properties of these new functional materials are being scrutinised. Electrochemistry is a suitable method for coupling particle activity to external circuitry. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: Nanoparticles; Q-dots; Self-assembly; Electrochemistry; Heterojunctions
1. Introduction Interest in the formation, characterisation and applications of nanoparticles continues unabated. The plot in Fig. 1 shows how the number of publications cited on the ISI database that contain the word nanoparticleŽs. in the title, representing only a fraction of the total papers published in the field, has increased since the middle of the 1980s. Initial studies of nanoparticles focused on their unique properties that stem from their high surface area to volume ratio and the spatial confinement of the charge carriers w1,2x. Recently, this work has been complemented by studies of functional materials formed by the controlled assembly of nanoparticles w3,4x. This article will describe the contributions of electrochemistry to recent advances in nanoparticle science in fields as diverse as optoelectronics and biological sensors. Electrochemistry is an enabling technology across the whole spectrum of nanoparticle research, from particle preparation to sensor applications and from bandgap determination to solar cell operation. After a look at how electrochemistry may
U
Tel.: q44-117-928-7668 fax: q44-117-925-0612. E-mail address: [email protected] ŽD. Jason Riley..
be employed in nanoparticle preparation the article will focus on how nanoparticles formed using colloid chemistry can be addressed electrochemically. Throughout the review the focus will be on how electrochemistry can aid studies of nanoparticles.
2. Electrodepsoition as a route to nanoparticles In colloid science nano-sized particles are often prepared from supersaturated solutions containing organic surfactants. The primary role of the organic ligands is to passivate the surface and suspend growth. This technique of nanoparticle preparation is termed arrested precipitation. Analogous methods for the formation of nanoparticles on conducting substrates have been developed in electrochemistry. It has been demonstrated that the addition of surface capping agents can lead to the deposition of nanoparticles during electroplating w5x. The additives serve to arrest particle growth and the strength of the absorption correlates to the particle size. A more general method to suspend particle expansion is the regulation of the electrodeposition parameters; current or voltage w6,7x. A two step procedure is employed, a high overpotential is applied for a short period to nucleate metal particles at the surface. Then to slowly grow the
1359-0294r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 9 - 0 2 9 4 Ž 0 2 . 0 0 0 4 7 - X
D.J. Riley r Current Opinion in Colloid & Interface Science 7 (2002) 186᎐192
Fig. 1. The number of publications on the ISI database each year that contain the word nanoparticleŽs. in the title.
particles to the required final dimension a low overpotential pulse is applied. The low overpotential promotes a modest variation in particle size, typically less than 7%, as it both prevents coupling of the diffusion layers of adjacent particles and results in a particle growth rate that decreases with increasing particle dimension. The shape of the particles produced by arrested electrodeposition depends on the substrate employed and the applied overpotential. Metal; including gold, silver and nickel w6,7x; and conducting polymer nanoparticles w8x with spherical geometry have been prepared on basal graphite planes. Also on graphite spherical nanoparticles are formed when a high overpotential is employed w9x. However, by careful selection of experimental conditions it is possible to form nanowires on graphite electrodes w10,11x. This has been achieved by selecting potentials that restrict electrodeposition to step edges. Palladium nanowires, as narrow as 55 nm in diameter and hundreds of m in length, prepared using this method, have been transferred to a polymer matrix and used as hydrogen sensors, the resistivity of the wires decreasing on exposure to H 2 w11x. Material growth using porous templates to control size and shape would appear to be a universal route to nanoparticles. However, for the majority of deposition routes, e.g. precipitation and chemical vapour deposition, problems arise due to the non-uniform distribution of product and the plugging of pores. These issues are circumvented in templated electrodeposition, see Fig. 2, as material growth can only take place at the pore base w12᎐14x. Template electrodes have been formed on a range of materials
187
including track-etched mica w15x and porous alumina membranes w16x. Templated electrodeposition has been used to prepare nanowires of a range of materials. For example, the methodology was employed to prepare suspensions of fluorescently labelled nickel wires, of nanometer radius and micron length, that allowed the direct visualisation of the effect of magnetic field on particle alignment w17x. In an ingenious development of the template synthesis technique patterned nanowires have been grown by periodically moving the working electrode between a solution containing gold ions and a solution of silver ions w18x. The contrast in reflectivity of the gold and silver sections yields wires that have been described as nano-barcodes. It is envisaged that in the future such nanorods will be used in bioassays, the barcode element allowing the chemical history of the particle to be traced. In an interesting variation of the above, templated electrodeposition has been employed to prepare high surface area material containing nanosized cavities w19,20x. Arrays of spherical polystyrene nanoparticles were crystallised on to a gold substrate from a colloidal sol. On electrodeposition of a metal on the resultant electrode a metal-polystyrene composite was formed. The polystyrene particles were then dissolved to yield a metal layer with nanosized voids. These cavities supported localised surface plasmons and resulted in sharp spectral features in the film reflectivity. The reflectivity of the film could be tuned by varying the size of the polystyrene particle employed in the deposition process w20x.
3. Charge transfer to and from Q-dots Particles that, as a result of the spatial confinement of charge carriers, display properties that differ from those of the chemically identical bulk material are termed Q-dots. The ability to tune the property of a Q-dot via control of its size has led many scientists to postulate that such particles will be the basis of the electronic devices of the future. To assess this assertion it is necessary to investigate the effects and kinetics of charge transfer at Q-dots. The difficulty of wiring the particles is an obvious obstacle to such studies. Whilst STM has allowed single particles to be studied w21᎐23x, a host of electrochemical techniques have been employed to investigate the electronic properties of arrays of nanoparticles. In the majority of such experiments the electrode and the electrolyte act as the ‘wires’ that couple the particle to the circuit. In electrochemical studies of electrolyte containing metal nanoparticles the size and dispersivity of the particles are key issues w24᎐26x. A metal typically has
188
D.J. Riley r Current Opinion in Colloid & Interface Science 7 (2002) 186᎐192
Fig. 2. Schematic illustrating the steps involved in the templated electrodeposition of nanowires.
a capacitance of tens of microfarads per cm2 and charging of the bulk material is continuous. However, for a metal Q-dot, that is stabilised by a dielectric layer of organic surfactant and is suspended in a solution of low dielectric constant, the capacitance, C, can be less than an attofarad. The energy required to add a single electron to such a particle, e r C, is greater than k B T r e, i.e. the charging of the particle is quantised. Therefore, on performing a potential scan on a solution containing metal Q-dots all of similar capacitance, i.e. a monodisperse size distribution, the discontinuity in particle charging leads to current peaks. Each peak corresponds to a single electron transfer event between the electrode and those metal particles near the surface. The separation of the peaks depends on C and hence, the particle size. As the rate of charge transfer is limited by transport of Q-dots to the electrode surface analysis of the current allows the diffusion coefficient of the particles to be calculated w25x. It is more usual to perform electrochemical studies
of charge transfer in Q-dot systems using self-assembled films of the nanoparticles. It has proved possible to deposit multilayers of suitably surface functionalised metal nanoparticles onto conducting substrates via ion bridging w27x. The resultant modified electrodes show evidence of quantised charging. Charge transport at such modified electrodes requires electrons to tunnel across the organic layers that separate the nanoparticles. Studies of the current time transient for film charging allows the rate of electron hopping to be determined. It is apparent that the rate of electron transfer between a nanoparticle and the substrate will be influenced by the organic moiety that separates the two. An organic chain with a redox active group offers a particularly elegant illustration of this point w28x. Gold nanoparticles were deposited on a gold electrode using a dithiol link, consisting of 20 methylene groups with a bipyridinium at the centre. An STM tip was employed to investigate ease of electron tunnelling between the nanoparticle and the substrate as the oxidation state of the
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bipyridinium group was electrochemically altered. The lowest resistance was obtained when the bipyridinium moiety was able to act as an electron shuttle. For metal Q-dots deconvolution of the current flowing at the electrode surface, the short circuit current, and that at the nanoparticle surface can prove a problem. This issue is easily avoided for electrodes modified with semiconductor Q-dots where photoelectrochemical studies yield signals that originate only from those nanoparticles that absorb the incident light. Simple photocurrent spectroscopy allows the absorption spectrum of the particles at a modified electrode interface to be measured, yielding information on the size of the Q-dots at the surface w29x. Using intensity modulated photocurrent spectroscopy, a technique in which the magnitude and phase of the photocurrent is measured relative to a sinusoidally modulated incident light intensity, the kinetics of charge transfer at semiconductor nanoparticle solution interfaces has been studied w30᎐34x. The influence of the length of the spacer molecule on the rate of electron transfer from CdSe Q-dots to a gold substrate has been studied w31x. The particles were linked to the electrode surface using a series of rigid disulfide molecules of varying length. The rates of electron transfer between the conduction band of the nanoparticle and the gold and the gold and trap states of the CdSe were both found to have an exponential dependence on the length of the spacer. The ability to control electron injection into the semiconductor Qdots allows the influence of charge on the optical properties of the particles to be investigated. Using CdS modified electrodes the bleaching of particle absorbance by electron injection has been studied w33,35x. The results obtained indicate that the change in absorbance is due to a Moss᎐Burstein shift in the bandedge as bleaching is only observed when potentials are reached for which it is thermodynamically favourable to inject electrons from the substrate to the Q-dot conduction band. Studies of semiconductor Q-dots may also be performed in the dark. In a recent paper w36x the electrochemistry of CdS Q-dots suspended in N, N⬘-dimethylformamide was reported. From the onsets of the anodic and cathodic currents the energies of the conduction band and valence band of the nanoparticle can be determined. The separation between the semiconductor bands as measured electrochemically matched the optical bandgap of the Q-dots. In the work described above the electrode acts as one of the contacts to the Q-dot. This need not be the case and it is possible to couple a nanoparticle to a circuit via electrolyte only. This has been achieved w37x using Scanning Electrochemical Microscopy ŽSECM.. A schematic of the technique as applied to nanoparticle layers is shown in Fig. 3. It is apparent
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Fig. 3. Schematic showing how SECM using methyl viologen as a probe may be employed to investigate the properties of gold nanoparticles deposited on an insulating substrate. Note that the components of the drawing are not to scale, typically the gold nanoparticles are less than 10 nm diameter and the ultramicroelectrode is 25 m in diameter w37x.
that the current flowing due to the methyl viologen dication reduction at the ultramicroelectrode is dependent on the rate of the reaction, Au
MVqq Hq ª MV 2qq H 2 , at the gold nanoparticles.
4. Assemblies of nanoparticles The fabrication of novel commercial devices based on assemblies of nanoparticles is the goal of much nanoparticle research. As discussed above electrochemistry offers an easy route of coupling nanoparticle activity to an external circuit and is therefore often key to the realisation of a working device. A major advantage of nanoparticle assemblies over conventional materials is the high surface area to volume ratio. For example, a nanocrystalline TiO 2 film of 10 m thickness may have an internal surface area approximately 1000 times greater than the geometric area. It is the high surface area that, when coated in dye, leads to the high absorbance and hence, efficient light harvesting of dye sensitized solar cells, a technology recently reviewed in this journal w38x. This idea of enhancing the optical absorption of a film by increasing the internal surface area has recently been exploited in a new generation of electrochromic displays w39᎐41x. It has been demonstrated that by attaching suitable redox active chromophores to transparent nanostructured TiO 2 and SnO 2 films
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Fig. 4. Schematic illustrating how a DNA sensor can be built using nanoparticle assembly. A photocurrent from the assembly indicates the presence of the analyte DNA. CdS1 and CdS2 are nanoparticles coated with DNA-1 and DNA-2, respectively w48x.
supported on conducting glass a display device with excellent performance characteristics can be fabricated. It is of note that the mechanism of chromophore oxidation at the TiO 2 surface is not yet fully understood. The high surface area of nanoparticle modified electrodes is also attractive in catalysis w42,43x and electrochemical sensor w44,45x applications. An example of the latter is the electroanalysis of proteins electrostatically attached to metal oxide surfaces w46,47x. The nanoparticulate surface not only enhances current due to the high surface area but the scale of surface roughness removes the requirement for a solution phase species to mediate electron transfer to the active redox site of the protein. A more difficult means of exploiting nanoparticles, that in the long term will probably prove more rewarding, is the development of devices that require a high degree of architectural integrity. Work in this area is in its infancy. So far, the alteration of surface groups to enhance nanoparticle functionality and the fabrication of semiconductor heterojunctions has been considered. A particularly illustrative example of the former is the DNA sensor w48x shown in Fig. 4. It is
apparent that the photoelectrochemical signal from the modified electrode depends on the amount of analyte present in solution. Enhancement of the signal in the sensor was attained by adding wRuŽNH 3 . 6 x 3q, it is postulated that the ions bind electrostatically to the DNA and mediate the transfer of photogenerated charge carriers. Charge transfer in sols of composite semiconductor nanoparticles has been extensively studied w49,50x, however, investigations of assemblies of mixed semiconductor nanoparticles are limited. A method of depositing CdS nanoparticles on nanocrystalline TiO 2 electrodes that permits control of particle size has recently been reported w51x. It was shown that photogenerated charge carriers are transferred from the CdS to the TiO 2 substrate, offering the possibility of building a sensitized solar cell without the need for an organic dye to absorb the light.
5. Conclusions For the past decade there has been interest in
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isolated nanosized particles as they possess novel physical and chemical properties. The recent realisation that the controlled assembly of nanoparticles can further enhance material attributes has led to increased activity in the field. The exploitation of Q-dots in devices requires that methods of transducing their activity are developed. Above we have demonstrated that electrochemistry offers an excellent means of contacting nanoparticles; allowing the influences of charge and potential on particle properties to be investigated and the development of sensor, electrochromic and solar cell devices in which electrochemistry is used to address nanoparticle arrays. In the future it is likely that electrochemistry will remain the method of choice for investigating the electronic properties of new materials. Whether, electrochemistry will remain a dominant technology as devices reach commercialisation will depend on whether the issues surrounding the use of liquid electrolytes can be addressed. Thus far the range of electrochemical methods employed in nanoscience has been limited. If electrochemistry is to remain at the forefront of this field advanced techniques must be employed in order to address some of the phenomena recently observed in Q-dot assemblies. For example, to study the ability of redox ions to mediate charge transfer from nanoparticles methods of decoupling the ion and nanoparticle electrochemistry will be required. Acknowledgements We are grateful to the Royal Society, Nuffield Foundation and EPSRC for their support of our research in the electrochemistry of nanoparticulate systems. References w1x Trindade T, O’Brien P, Pickett NL. Nanocrystalline semiconductors: Synthesis, properties, and perspectives. Chem Mat 2001;13:3843᎐3858. w2x Zhang JZ. Interfacial charge carrier dynamics of colloidal semiconductor nanoparticles. J Phys Chem B 2000;104: 7239᎐7253. w3x Fendler JH. Chemical self-assembly for electronic applications. Chem Mat 2001;13:3196᎐3210. w4x Shipway AN, Willner I, Nanoparticles as structural and functional units in surface- confined architectures. Chem Commun 2001; 2035᎐2045. w5x Mastai Y, Gal D, Hodes G. Nanocrystal-size control of electrodeposited nanocrystalline semiconductor films by surface capping. J Electrochem Soc 2000;147:1435᎐1439. w6x Liu H, Favier F, Ng K, Zach MP, Penner RM. Size-selective electrodeposition of meso-scale metal particles: a general method. Electrochim Acta 2001;47:671᎐677. w7x Penner RM. Mesoscopic metal particles and wires by electrodeposition. J Phys Chem B 2002;106:3339᎐3353. w8x Tang ZY, Liu SQ, Wang ZX, Dong SJ, Wang EK. Electrochemical synthesis of polyaniline nanoparticles. Electrochem Commun 2000;2:32᎐35.
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w9x Zach MP, Penner RM. Nanocrystalline nickel nanoparticles. Adv Mater 2000;12:878᎐883. w10x Zach MP, Ng KH, Penner RM. Molybdenum nanowires by electrodeposition. Science 2000;290:2120᎐2123. w11x Favier F, Walter EC, Zach MP, Benter T, Penner RM. Hydrogen sensors and switches from electrodeposited palladium mesowire arrays. Science 2001;293:2227᎐2231. w12x Sun L, Searson PC, Chien CL. Finite-size effects in nickel nanowire arrays. Phys Rev B 2000;61:R6463᎐R6466. w13x Sapp SA, Mitchell DT, Martin CR. Using template-synthesized micro- and nanowires as building blocks for self-assembly of supramolecular architectures. Chem Mat 1999;11: 1183᎐1185. w14x Davydov DN, Haruyama J, Routkevitch D et al. Nonlithographic nanowire-array tunnel device: Fabrication, zero-bias anomalies, and Coulomb blockade. Phys Rev B 1998;57: 13550᎐13553. w15x Sun L, Chien CL, Searson PC. Fabrication of nanoporous single crystal mica templates for electrochemical deposition of nanowire arrays. J Mater Sci 2000;35:1097᎐1103. w16x Masuda H, Ohya M, Asoh H, Nishio K. Photonic band gap in naturally occurring ordered anodic porous alumina. Jpn J Appl Phys Part 2 2001;40:L1217᎐L1219. w17x Tanase M, Bauer LA, Hultgren A et al. Magnetic alignment of fluorescent nanowires. Nano Lett 2001;1:155᎐158. w18x Nicewarner-Pena SR, Freeman RG, Reiss BD et al. Submicrometer metallic barcodes. Science 2001;294:137᎐141. w19x Bartlett PN, Birkin PR, Ghanem MA, Toh CS. Electrochemical syntheses of highly ordered macroporous conducting polymers grown around self-assembled colloidal templates. J Mater Chem 2001;11:849᎐853. w20x Coyle S, Netti MC, Baumberg JJ, Ghanem MA, Birkin PR, Bartlett PN, Whittaker DM, Confined plasmons in metallic nanocavities. Phys Rev Lett 2001, 8717:art. no.-176801. w21x Alperson B, Rubinstein I, Hodes G, Identification of surface states on individual CdSe quantum, dots by room-temperature conductance spectroscopy. Phys Rev B 2001, 6308:art. no.-081303. w22x Bakkers E, Hens Z, Zunger A et al. Shell-tunneling Spectroscopy of the single-particle energy levels of insulating quantum dots. Nano Lett 2001;1:551᎐556. w23x Bakkers E, Vanmaekelbergh D. Resonant electron tunneling through semiconducting nanocrystals in a symmetrical and an asymmetrical junction. Phys Rev B 2000;62:R7743᎐R7746. w24x Zamborini FP, Gross SM, Murray RW. Synthesis, characterization, reactivity, and electrochemistry of palladium monolayer protected clusters. Langmuir 2001;17:481᎐488. w25x Chen SW, Sommers JM. Alkanethiolate-protected copper nanoparticles: Spectroscopy, electrochemistry, and solid-state morphological evolution. J Phys Chem B 2001;105:8816᎐8820. w26x Chen SW, Templeton AC, Murray RW. Monolayer-protected cluster growth dynamics. Langmuir 2000;16:3543᎐3548. w27x Zamborini FP, Hicks JF, Murray RW. Quantized double layer charging of nanoparticle films assembled using carboxylaterŽCu2q or Zn2q .rcarboxylate bridges. J Am Chem Soc 2000;122:4514᎐4515. w28x Gittins DI, Bethell D, Schiffrin DJ, Nichols RJ. A nanometre-scale electronic switch consisting of a metal cluster and redox-addressable groups. Nature 2000;408:67᎐69. w29x Torimoto T, Tsumura N, Nakamura H et al. Photoelectrochemical properties of size-quantized semiconductor photoelectrodes prepared by two-dimensional cross-linking of monodisperse CdS nanoparticles. Electrochim Acta 2000;45:3269᎐3276. w30x Bakkers E, Roest AL, Marsman AW et al. Characterization of photoinduced electron tunneling in goldrSAMrQ-CdSe
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