Electrochemistry Communications 23 (2012) 87–89

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Electrochemistry Communications

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Photocatalytically initiated electroless deposition of macroporous metal films ontoinsulating substrates

Michael A. Bromley, Colin Boxall ⁎Lancaster University, Engineering Department, Lancaster LA1 4YR, UK

⁎ Corresponding author. Tel.: +44 1524 593109; fax:E-mail addresses: m.bromley@lancaster.ac.uk (M.A.

c.boxall@lancaster.ac.uk (C. Boxall).

1388-2481/$ – see front matter © 2012 Elsevier B.V. Alldoi:10.1016/j.elecom.2012.07.015

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 June 2012Received in revised form 13 July 2012Accepted 16 July 2012Available online 22 July 2012

Keywords:Electroless depositionNanostructured metalNanoporous metalPhotocatalysisTiO2

By using photocatalytically initiated electroless deposition (PIED)we have depositedmacroporousmetal filmswith highly ordered arrays of sub-μm (hemi)spherical pores directly onto the surface of insulating substratesfor the first time. This has been achieved by sensitisation of the target substrate with a TiO2 photocatalystfollowed by PIED of the target metal into the interstitial spaces of a template composed of hexagonally closepacked polystyrene microspheres.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Metal materials with an ordered, sub-μm scale pore structure inter-faced directly with an insulating substrate are attractive for use in cleanenergy production (fuel cells [1]), energy storage [2,3], sensors (especiallysurface enhanced resonance Raman techniques [4,5]) and separationscience [6]. Metal layers, including structured metal layers, are most con-veniently and cost effectively formed by electrodeposition although thisremains applicable only to conducting substrates. From a processingperspective, it would be convenient if an electroless deposition-basedfabrication route existedwhereby porousmetals could be formed directlyonto insulator surfaces. To date, no such route exists.

In contrast, a number of routes exist by which such porous metalsmay be deposited onto the surfaces of conductors. Of these, the mostpractical and simple is perhaps the method first reported by Bartlettet al. [7] wherein metal films with ordered arrays of sub-μm sphericalvoids are prepared by electrodeposition into the interstitial spacesbetween polystyrene (PS) spheres assembled on metal electrode sur-faces. The spheres are then removed, by dissolving them in THF ortoluene, to leave a porous metal film with regular inter-connectedcavities. Using our recently reported insulator metallisation processof photocatalytically initiated electroless deposition (PIED) [8–10],we have developed a novel method that allows the deposition ofsuch porous films onto non-conducting substrates, something we be-lieve to be a world first.

+44 1524 381707.Bromley),

rights reserved.

The metallisation of non-conducting materials is typically per-formed through electroless plating. While independent of electricalcurrent, electroless plating does require extensive and repetitive sen-sitisation of the substrate surface with expensive Sn–Pd catalysts be-fore metal ion reduction can take place [11]. Semiconductor materialssuch as TiO2, SnO2, and WO3 can act as photocatalysts, generatingelectron–hole pairs upon the absorption of ultra-band gap light ener-gy. As described in Ref. [8], we have appropriated the photogeneratedelectrons for metal ion reduction within the PIED process, producinga metallisation technique that is both more controllable and cost-effective than conventional electroless deposition [8–10]. Metallisationthrough PIED is comprised of two main steps:

(i) The semiconductor was irradiated with ultra-band gap lightenergy (λ=312 nm) to generate conduction band electrons.These electrons reduce metal precursor ions in the solution,forming metal nuclei on the semiconductor surface, Fig. 1 (i).

(ii) Once sufficient nucleation sites have been photocatalyticallygenerated, these begin to grow independently of irradiationthrough an auto-catalytic electroless deposition reaction, drivenby the electroless plating bath reductant, Fig. 1 (ii), the growingnuclei eventually coalesce to form a coherent metal layer. Thissecond step can occur both in the presence and absence of light,the latter being particularly advantageous for producing thick,non-transparent metal layers.

As mentioned above, the resulting metal film has no artificiallyinduced structure. However, this paper is concerned with the deposi-tion of metals with highly ordered nano-scale porosity by PIED, some-thing that we have achieved through the assembly of a polystyrene

Fig. 1. Two stages of PIED: (i) photoreductivemetal nucleation on irradiated photocatalyst;(ii) auto-catalytic electroless deposition process.

88 M.A. Bromley, C. Boxall / Electrochemistry Communications 23 (2012) 87–89

microsphere template at the photocatalyst-sensitised insulator surfacein a manner similar to that described by Bartlett et al. for templatedelectro-deposition (as opposed to electroless deposition) [7].

2. Experimental

PIED of non-porous metals has been described in detail in a previ-ous communication from this laboratory [8] to which the interestedreader is referred. Here we present a summary of the process andthe modifications made to induce a porous structure in the depositedmetal. For PIED to occur, the surface of the insulating substrate tobe metallised is first sensitised with a photocatalyst, such as TiO2.This is applied by either (i) sol–gel spin coating and annealing [12]; or(ii) spin or dip coating of commercially available TiO2 nanoparticles,in each casewithout the use ofmolecular linkers required during photo-catalytic patterning techniques [13]. The chemically inert mesoporousTiO2 layer remains on the substrate post-metallisation though causesno adverse physical or chemical effects.

Microsphere templates are self-assembled on the substrate sur-face to produce a hexagonally close-packed (HCP) array [14]. Briefly,this is achieved by an incubator-controlled evaporation of a thin layerof a 1 wt.% suspension of 1 μm diameter PS spheres, a process thatrequires the microsphere suspension to spread across the surface.Here, the presence of a sensitising TiO2 layer on the substrate surfaceoffers further process advantage; under UV irradiation, TiO2 is ren-dered super-hydrophilic, resulting in water drop spreading [15,16].Hence, 30–60 min of UV irradiation prior to application of the micro-sphere suspension was found to greatly assist in microsphere arrayassembly.

Fig. 2. SEM images of (i) photo-generated Ag nucleation; (ii) PIED generated Ag layer with aafter template removal (iv) nanoporous Pd deposit on glass after template removal, deposideposit; (vi) multi-layer porosity in Pd.

TiO2 sensitised substrates, either with or without microspheretemplates, are immersed in a freshly prepared electroless platingsolution, consisting of a metal precursor, complexant, stabilizer andreducing agent [8,10] within a UV-transparent quartz reaction vessel.Metal deposition is initiated by irradiation with 6×8 W UV lampsarranged in a hemi-cylindrical array facing towards the target sub-strate. Solutions are purged with N2 for 20 min before irradiationand throughout the deposition process.

Post deposition, substrates are simply rinsed with doubly deionisedwater (DDW) and dried in air. The microsphere template is thenremoved by dissolution in toluene.

3. Results and discussion

Using PIED, both Ag and Pd have been deposited around 1 μm di-ameter PS microsphere templates on TiO2 sensitised glass substrates.SEM images of such deposits pre- and post-template removal areshown in Fig. 2. For both metals, the films can be seen to be coherentand to be conductors (surface resistivity, Rs, ~0.2 Ω/sq), demonstrat-ing both 1 and 2 electron reduction processes. Deposited films arestrongly adhered to the substrate, demonstrated by compliancewith the adhesion test BS-EN-ISO-2819:1995.

Whilst the general mechanism by which PIED occurs, Fig. 1, wouldbe expected to be the same on templated and non-templated sub-strates, the rate of metal deposition on the former is found to bereduced with respect to the latter. Thus, whilst a period of 15 min issufficient to produce a non-porousmetal film that conducts electricitywith an Rs of 0.2 Ω/sq, 60–120 min is required to achieve the same Rs

with a template in place. This suggests that the template acts as apartial physical barrier, restricting access of the initiating UV irradia-tion and electroless plating bath constituents to the TiO2 beneath.However, the regular interstitial spaces of the HCP microspherearray allow sufficient access to the underlying semiconductor forPIED to occur. An initial photocatalytic stage drives metal nucleationdirectly onto the TiO2 sensitised substrate, Fig. 2 (i), and subsequentauto-catalytic deposition continues themetal growthupwardly through-out the aforementioned interstices, Fig. 2 (ii), forming a metal filmaround the base of the microsphere template.

Post template-removal, the resultant metal film displays orderednano-scale porosity consisting of hemispherical voids mirroring the

1 μm microsphere template still present, deposition time=60 min; (iii) same deposittion time=60 min; (v) 3D ‘shape from shading’ surface reconstruction of the same Pd

89M.A. Bromley, C. Boxall / Electrochemistry Communications 23 (2012) 87–89

shape of the now removed microspheres, Fig. 2 (iii) and (iv). At thebase of each hemispherical void lies a smaller hole which is opento the substrate below and forms at the point of contact betweenthe microspheres and the substrate surface. In the case of ∅=1 μmmicrospheres, each hole has ∅b200 nm.

Throughout the sample, the porosity maintains the hexagonal ar-rangement of the microsphere array and, in the most ordered regions,pore centre to centre separation can be measured to 1 μm±10 nm.This is consistent with the size of the microspheres used and demon-strates that the template is not affected by the PIED process, retainingoriginal size and position, and also that no metal shrinkage or expan-sion occurs post-deposition or during template removal. Particularlynoticeable when viewed 3-dimensionally, Fig. 2 (v), is the consistentheight of the upper metal surface across the sample, indicative ofcontrolled and homogenous growth. The metal layer thickness is rep-resented by the sagitta of each hemispherical pore. Using the diame-ter of the templating microsphere and the measured diameter of thepore upper opening, the sagitta, and layer thickness, is found to be137 and 120 nm for the Ag and Pd samples of Fig. 2, each with adeposition time of 60 min, the thickness of the metal deposit beingaffected by the deposition time. As the thickness of deposits producedwith irradiation throughout the deposition is similar to those pro-duced through part irradiated, part dark deposition periods [8], theoverall deposition period can be used as a control parameter analo-gous to the use of coulombic charge passed during electrodeposition.

Where the template consists only of a monolayer of microspheres,the thickness of the deposited metal must be restricted to less thanthe diameter of one microsphere. If metal is deposited to a thicknessgreater than this, the nanoporosity is occluded as the metal growthspreads over the upper surface of the microspheres. We have foundthat the use of a multi-layer microsphere template allows for thedeposition of layers with a thickness greater than one microspherediameter and results in a metal film with an interlinked, multilayernanoporosity, Fig. 2 (vi).

Demonstrating its utility for a range of insulating substrates, PIED hasalso been used to deposit porous Ag, Fig. 3, and Pd onto nanoparticulateTiO2 sensitised 0.2 μm pore size polyvinylidene fluoride (PVDF) mem-branes. In both instances, the deposited metal layers are found to beconductors and of metallic appearance. However, the roughness ofthe substrate surface partially disrupts the microsphere array self-assembly; as a result, the templates show less two-dimensional orderthan those produced on smooth glass substrates although they remainsuitable for the induction of widespread porosity in the depositedmetal film.

Similarly, the holes at the pore base, formed at the contact pointsbetween the spheres and substrate, are less consistent in shape anddiameter, ranging from b50 nm to 250 nm, attributable to an irregularcontact area between microspheres and the rough PVDF surface. How-ever, spatially consistent metal deposition is observed; the upper open-ing diameters of the hemispherical pores is measured at 690 nm±

Fig. 3. Nanoporous Ag on PVDF deposited to a thickness of (i) 138 nm in 60 min and(ii) 344 nm in 120 min.

25 nm after 60 min deposition time, Fig. 3 (i), and 950 nm±25 nmafter 120 min, Fig. 3 (ii). These equate to a metal film thickness of138 nm and 344 nm respectively, demonstrating that, as on glass,thicker layers are formed with increased deposition time. We havefound that deposition can be applied to a single side of the membraneby selective sensitisation through spin coating of the TiO2, or on bothsides by full sensitisation through dip coating. In the case of the latter,no throughmembrane deposition or conductivity is observed, providingtwo individual porousmetal films on either side of the samemembrane.Thismay be applicable to thin layer polymer electrolyte fuel cells or elec-trochemically assisted membrane separation processes.

4. Conclusion

We have presented the first reported electroless deposition ofmetals with highly ordered nanometer scale porosity on insulating sub-strates. We also provide the first reported use of electroless depositionwith a controllable onset in the creation of ordered nanoporous metalson any kind of support, be it insulator, semiconductor or metal.

The novel PIED process allows for controllable photo-initiation ofthe metallisation process whilst facilitating continued auto-catalyticelectroless deposition to produce coherent porous metal films, thethickness of which is determined by the total deposition time. Theresulting metal deposits can be induced with single or multi-layernano-scale porosity with a highly regular and predictable order withcoverage and pore diameter beingdetermined by the spatial applicationandparticle diameter of the template array. This has been achievedwitha variety of metals and onto both organic and inorganic insulatorsubstrates.

Insulating materials metallised with PIED have potentially wideapplications in membrane and analytical separation technology, desa-lination, electrode/solid electrolyte composites for fuel cells, energystorage and sensors—especially surface enhanced resonance Ramanspectroscopy (SERRS).

Acknowledgement

We thank the Royal Society of Chemistry UK, EPSRC (Award NoEP/I002928/1) and The Lloyds Register Educational Trust (LRET) forfinancial support. The LRET is an independent charityworking to achieveadvances in transportation, science, engineering and technology educa-tion, training and research worldwide for the benefit of all.

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