Microporous and Mesoporous Materials 153 (2012) 131–136

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Microporous and Mesoporous Materials

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Gold nanotube membranes have catalytic properties

Yang Yu a, Krishna Kant a,b, Joe G. Shapter b, Jonas Addai-Mensah a, Dusan Losic a,⇑a University of South Australia, Ian Wark Research Institute, Mawson Lakes Campus, Mawson Lakes, Adelaide, SA 5095, Australiab Flinders University, School of Chemical and Physical Sciences, Bedford Park, Adelaide, SA 5042, Australia

a r t i c l e i n f o

Article history:Received 20 August 2011Received in revised form 4 December 2011Accepted 5 December 2011Available online 13 December 2011

Keywords:Gold nanotube membranesAnodic aluminum oxide membranesCatalytic membranesElectroless gold depositionTemplate synthesis

1387-1811/$ - see front matter Crown Copyright � 2doi:10.1016/j.micromeso.2011.12.011

⇑ Corresponding author. Tel.: +61 8 8302 6862; faxE-mail address: dusan.losic@unisa.edu.au (D. Losic

a b s t r a c t

This work presents the fabrication of gold nanotube membranes (GNT) using gold deposition on porousanodic aluminum oxide (AAO) template and demonstrates the catalytic properties generated by thenanostructured gold surface. AAO membranes were prepared by electrochemical anodization of Al in0.3 M oxalic acid electrolyte. Electroless gold deposition on AAO was performed via several steps usingcommercially available gold plating solutions. Scanning electron microscopy (SEM) images confirm thatthe gold nanotubes formed inside AAO pores have characteristic nanoclustered morphology as a result ofthe nucleation process during gold deposition. It was found that catalytic properties of GNT membranesdepend on the size of gold nanoclusters which can be controlled by pH during nucleation process. Theexcellent catalytic properties (catalytic rate constant k = 0.132 min�1) of the GNT membranes were dem-onstrated by testing catalytic conversion of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP) in the pres-ence of NaBH4 as a reductant. The correlation between the size of gold nanoclusters and catalytic activitywas verified showing the capability of controlling and further improving catalytic properties of GNTmembranes.

Crown Copyright � 2011 Published by Elsevier Inc. All rights reserved.

1. Introduction

Gold is the noblest of metals with remarkable resistance to oxi-dation and until recently it has been widely believed that gold istoo inert to be useful as a catalyst [1]. Although smooth gold sur-faces are chemically and catalytically inert, nanosized (<5 nm) goldparticles have been found to be very effective for a number of cat-alytic reactions [2,3]. During the last two decades, the morpholog-ical control of gold micro/nano materials has drawn great interestbecause their shape and structures are closely related to their unu-sual catalytic, electronic and optical properties [3,4]. Catalyticactivities of gold nanostructures have become widely recognizedfor various reactions such as CO oxidation, epoxidation of propene,selective or partial oxidation of methanol and water–gas shift reac-tions [5]. Whilst the reasons for these nanostructures do have ef-fect on catalytic properties of gold is still widely debated, there isan agreement that the metastable (or high energy) surface of goldatoms are involved in catalytic process [2,6].

Several studies have demonstrated that the catalytic propertiesof supported gold may change markedly depending on the particlesize, the nature of metal oxide supports, the interaction betweengold and the supporting oxide as well as the nanostructures of the ac-tive sites [7]. It was found that smaller Au nanoparticles (NPs) tend toshow higher catalytic activities as they have much greater surface to

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: +61 8 8302 3683.).

volume ratio [3,4]. However, smaller AuNPs aggregate easily mini-mizing their surface area and surface energy, resulting in a remark-able reduction in their catalytic activities. To overcome thesedisadvantages, increasing efforts have been devoted to the introduc-tion of AuNPs on/into less expensive solid supports such as poly-mers, carbon, metal and metal oxides to form composite catalysts[5,8–10]. AuNPs have been assembled on many supports with differ-ent nanostructures, including thin films, spheres, fibers, dendrimers,mesoporous silica, microporous metal–organic framework (MMOF)[8–11]. Although the assembly of AuNPs inside membranes withporous and tubular morphologies was demonstrated, their catalyticproperties were not widely explored [12].

Gold nanotube (GNT) membranes, prepared by electroless golddeposition using a polymer or porous anodic aluminum oxide(AAO) template were pioneered by Martin and successfully usedfor numerous applications including selective molecular separa-tion, enzyme reactors, drug delivery, biosensing and electroanaly-sis [13–18]. It was observed that the GNT membranes havecharacteristic nanostructured gold surfaces across the whole nano-tube structures as a result of specific crystal growth process duringchemical deposition of gold but these features have not drawn anyresearch attention [17,19]. We recently used the Raman spectros-copy to probe GNT membranes with clustered gold surfaces mod-ified with self-assembled monolayers (3-mercaptobenzoic acid)and confirmed their surface enhanced Raman scattering (SERS)characteristics [20]. The characteristic optical properties of GNTmembranes based on their unique tubular morphology and

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132 Y. Yu et al. / Microporous and Mesoporous Materials 153 (2012) 131–136

clustered gold surfaces have been demonstrated for the first timeshowing their potential for sensing and biosensing applications[20]. The high surface area of GNT membranes and their nanoclu-stered morphology are particularly attractive features of GNTswhich can be exploited to develop multifunctional membranes.

The fabrication of GNT membranes and the demonstration theircatalytic properties are presented in this paper. The schematic dia-gram of fabrication process and typical morphology of GNT mem-branes prepared by electroless gold deposition onto AAO is shownin Fig. 1. The internal structure of gold nanotubes was decoratedwith gold nanoclusters as a result of gold nucleation process duringthe growth of gold film. Catalytic properties of the prepared GNT–AAO composite membranes were evaluated using catalytic reduc-tion of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with sodiumborohydride (NaBH4) as a reductant. This is known as the bench-mark model reaction for evaluating the catalytic activities of vari-ous metal nanoparticles such as Au, Ag, Pt, and Pd [7,21]. The sizedependence of the catalytic properties of the GNT–AAO mem-branes was examined with different size gold structures obtainedby changing the nucleation process employed during fabrication.

2. Experimental section

2.1. Materials and chemicals

A high purity (99.997%) aluminum foil (0.1 mm) supplied fromAlfa Aesar (USA) was used as the substrate material. The gold plat-ing solution (Oromerse Part B) was purchased from Technic, Inc.(USA). 4-NP, sodium borohydride, tin(II) chloride, trifluoroaceticacid, nitric acid, ammonia, methanol, formaldehyde, sodium sul-fite, sodium bicarbonate, sulfuric acid, and ethanol were suppliedfrom Sigma–Aldrich (Australia) and used as received. Silver nitratewas obtained from Proscitech (Australia). Milli-Q water (18 MX)was used for rinsing and preparation of all solutions.

2.2. Fabrication of porous anodic aluminum oxide (AAO) membranes

AAO membrane (1 cm in diameter) was prepared using electro-chemical anodization of Al using previously described procedures[22,23]. The Al foil was cleaned in acetone and then electrochem-ically polished in a 1:4 volume mixture of HClO4 and ethanol witha constant voltage of 20 V for 2 min to achieve a mirror finishedsurface. Two-step anodization was performed using an electro-chemical cell equipped with a cooling stage held at a temperatureof �1 �C. The first anodization step was performed under 40 V for6–8 h in 0.3 M oxalic acid (H2 C2O4). Afterwards, the formed porousoxide film was chemically removed by a mixture of 6 wt.% of H3PO4

and 1.8% chromic acid for a minimum of 3–6 h at 75 �C. The secondanodization was then performed under 60–70 V in 0.3 M oxalic

Fig. 1. Schematic diagram of the fabrication of gold nanotube (GNTs

acid for 20–30 min to achieve AAO thickness about 30 lm. Theremaining underlying aluminum was then removed by a CuCl2/HCl solution to form AAO with exposed barrier film. The removalof the barrier layer of AAO was performed using a 5 wt.% H3PO4.

2.3. Fabrication of gold nanotube/porous anodic aluminium oxide(GNT–AAO) composite membrane

The gold coating of fabricated AAO membranes was performedby adapted electroless gold deposition described by Martin androutinely used for fabrication of gold nanotube membranes onpolymer and porous alumina templates [13,24–26]. The AAO mem-brane was incubated with 35% hydrogen peroxide solution over24 h to increase density of oxygen groups on the surface. In thefirst step called sensitization, the AAO was immersed into a solu-tion of 0.026 M SnCl2 and 0.07 M trifluoroacetic acid for 45 min fol-lowed by rinsing in Milli-Q water. The sensitized AAO was thenimmersed into 100 cm3 ammoniacal solution of 0.029 M AgNO3

for 30 min. In this step, a redox reaction leads the formation of athin layer of silver nanoparticles on the membrane surface to formcatalytic sites for the oxidation of formaldehyde and the concur-rent reduction of Au(I) to Au(0). In the third step called displace-ment deposition, or plating, the silver coated membrane waswashed with Milli-Q water and then was immersed into a standardgold plating solution (Oromerse Part B, USA) containing 0.079 MNa3Au(SO3)2, 0.127 M Na2SO3, 0.625 M formaldehyde and0.025 M NaHCO3 (electroless deposition step). The pH was ad-justed to pH = 8 and pH = 9 in two sets of preparation experimentsand the temperature was kept at �1 �C for the required depositiontime (24 h). After deposition, the samples were thoroughly rinsedseveral times with superfluous amount of Milli-Q water.

2.4. Characterization of catalytic activity of GNT–AAO membranes

The reduction of 4-NP to 4-AP by NaBH4 was used to determinethe catalytic activity of prepared GNT–AAO membrane adapting apreviously reported procedure [11,21]. A typical experiment wascarried out as follows: a reaction mixture of 50 cm3 of 4-NP andNaBH4 aqueous solution was first put into a 150 cm3 beaker witha magnetic stirrer and a cover glass, keeping the final concentra-tion of 4-NP at 3.91 � 10�6 M and NaBH4 at 3.91 � 10�3 M. TheBH�4 concentration selected for this experiment was in large excesscompared to that of 4-NP, making the reaction independent ofborohydride concentration. Then one GNT–AAO membrane(weight: �50 mg) was fixed to the beaker by a plastic holder anddipped into the solution. The catalytic activity was monitored viaabsorbance measurements (Cary 1E UV–Vis spectrophotometer)by recording the absorption spectra at 399 nm (attributed to thetypical absorption of 4-NP) of the prepared solution after contact

) membranes using anodic aluminum oxide (AAO) as template.

Y. Yu et al. / Microporous and Mesoporous Materials 153 (2012) 131–136 133

with the GNT–AAO membrane. Absorbance spectra were recordedevery 1 min, and the decrease of absorbance peak was used to eval-uate the catalyst’s activity. After the reaction, the GNT–AAO mem-brane was rinsed by Milli-Q water 5–10 times and stored in water.A series of diluted solutions of 4-NP and NaBH4 were also made todetermine the reactant’s concentration as a function of the absor-bance at 399 nm.

2.5. Structural characterization

Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDAX) (XL-30, Philips) were used to performstructural characterization and determine the chemical composi-tion of prepared membranes. Membranes samples from all the fab-rication steps were mounted on a double-faced adhesive tape, andcoated with ultra-thin platinum layer (3 nm) before imaging. Crosssection images were obtained by fracturing the membranes. In or-der to view the gold nanotubes formed within the pores, the AAOtemplate was removed by dissolution in 5 wt.% NaOH.

3. Results and discussion

A series of cross-sectional SEM images (Fig. 2(a) and (c)) showsthe products of the fabrication process of GNT membranes usingAAO membrane as a template. High resolution SEM images of indi-vidual pores from AAO template and corresponding gold nanotubestructures are presented to illustrate the changes in their morphol-ogy during this process. Typical topography of prepared AAO mem-branes was observed with an array of highly ordered and verticallyaligned pore structures, controllable pore diameters, length andshapes providing the ability to design GNT membranes with tun-able selectivity and transport properties [27]. Pores about 80–100 nm in diameter were revealed (Fig. 2(a)). SEM images of frac-tured GNT–AAO structures taken after the gold deposition show

Fig. 2. SEM images of (a) AAO membrane before; (b) after electroless deposition of goldremoval of the AAO template. Insets show high resolution images of pores and correspondafter gold deposition on AAO.

that gold was deposited inside AAO pores with formation of goldnanotube structures (Fig. 2(b and c)). EDAX analyses of AAO mem-branes before and after gold deposition show the purity of the goldstructures formed in pores as only the predominant peak for gold(E = 2.11 keV) was observed (Fig. 2(d)). The free-standing GNTswere obtained after completely dissolving the AAO template whichclearly reveals that the morphology and structure of the AAOmembrane is entirely preserved.

Higher resolution SEM characterization was performed to re-veal more structural details from gold nanotube structures(Fig. 3(a) and (d)). Comparative SEM images of the GNT top surfaceand AAO pores before deposition (Fig. 3(a) inset) indicate the aver-age pore outer diameter of nanotube structures is around 100 nmand the wall thickness is about 40 nm. Specifically, numerous goldnanoclusters with average particle sizes of 6.5 nm ± 4 nm andirregular forms were observed on the top surface of GNTs(Fig. 3(b)). These structures represent final stage of gold nucleationprocess and the same structured surface should be observed insideof nanotubes. To confirm this hypothesis SEM images of bothexternal and internal surface of gold nanotubes were obtained(Fig. 3(c and d)). The GNTs obtained after the removal of AAO tem-plate (Fig. 3(c)) also show nanoclustered surface on the externalwall with similar morphology seen on the top surface. These goldclusters originated from initial gold nuclei sites on the AAO surfaceformed on silver activation sites which formation are crucial forthe subsequent growth of the gold film and hence these initialcluster do influence the final morphology of the film obtained. Toconfirm the presence of gold clusters inside gold nanotubes, highresolution SEM image of fractured GNTs were obtained (Fig. 3(d))indicating slightly smaller structures (5 nm ± 3 nm) than observedon external surface. These results clearly prove that the internalsurface of the GNTs is composed of clustered morphology whichis schematically outlined in Fig. 3(e). Based on nanoscale dimen-sions of these gold clusters and high surface area of GNT

with gold nanotube structures created inside pores and (c) GNT membranes aftering gold nanotube structures. Bar scale in inset is 200 nm; (d) EDAX spectrum taken

Fig. 3. High resolution SEM images showing morphology of the prepared GNT structures; (a) the top view of GNT–AAO membranes showing clustered top surface. Thesurface of AAO pores before deposition is shown in the inset; (b) high-resolution image of single pore with gold clusters (arrows) about 5 nm in diameter; (c) GNT structureafter removal of AAO showing their intake structure across whole membrane; (d) details of fractured gold nanotube structure with clustered gold surface (arrows) inside ofnanotubes; (e) schematic diagram of single GNT–AAO structure (top and profile) which shows gold clusters on inner and outer nanotube surface (the AAO layer is onlypartially presented).

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membranes this substrate should have superior catalytic proper-ties in comparison with gold clustered planar surfaces.

Previous studies suggested that the nucleation and gold growthprocess during electroless deposition is dependent on pH, temper-ature, gold concentration and deposition time. Influence of theseparameters on the third (the displacement deposition) and fourthstep (gold electroless deposition) during plating are determiningparameters for nucleation site distribution, gold deposition rateand grain size. Among these factors, the pH is considered to havesignificant influence on gold growth kinetics and nucleation rateand therefore can be used as simple method to tailor the sizeand density of gold nanoclusters [19,28]. It has been shown thatthe growth rate at lower pH decreases and significantly increasesat higher pH. Therefore, GNTs were prepared with two differentpH conditions to investigate influence of the nanostructure on cat-alytic properties of GNT membranes (Fig. 4). High resolution SEMimages suggest differences in surface morphology of prepared goldnanotubes at different pH showing higher density and smaller goldclusters at lower pH (Fig. 4(a and b)) than at higher pH (Fig 4(d ande)). The supporting schemes to explain the formation of differentgold clusters are presented in Fig. 4(c and f). These results confirmthat slower growth rate at lower pH (pH = 8) generates smallergold clusters and the faster nucleation rate at higher pH (pH = 9)produces larger gold clusters, which is in agreement with previouswork [19,29,30].

An aqueous solution of 4-NP has a distinct UV–Vis spectral profilewith an absorption maximum at 399 nm. Absorbance spectra re-corded during the reduction of 4-NP to 4-AP by NaBH4 reaction wereused to evaluate catalytic activity of the prepared GNT–AAO mem-branes. Fig. 5 shows a typical UV–Vis absorption change of a solutioncontaining 4-NP (3.91 � 10-6 M) and NaBH4 (3.91 � 10-3 M), in con-tact with the GNT–AAO membrane clearly demonstrating a signifi-

cant decrease of adsorption of 4-NP which means �99.9%completion of reaction within 30 min. A new absorption peak simul-taneously appears at 297 nm, indicates the reduction was takingplace with the concentration of 4-AP product increasing as the reac-tion proceeds. The reduction does not occur in the absence of GNT–AAO membranes under the same experimental condition over 72 h,indicating that GNTs acted as a catalyst to facilitate the transfer ofelectrons from BH�4 to 4-NP. Since the BH�4 was the excess reactant,only the concentrations of 4-NP and 4-AP will influence the reactionkinetics. Therefore a pseudo-first-order kinetics was applied to the4-NP initial and instantaneous (Ct) concentration data for the evalu-ation of rate constants [8].

Fig. 6 shows C (lmol L�1) and ln(Ct/C0) versus time graphs forthe reduction of 4-NP using GNT–AAO membranes prepared at dif-ferent pH. The graph shows that the catalytic reaction over GNT–AAO membranes prepared at high pH (larger Au clusters) was al-most complete within 45 min in the presence of NaBH4, but a sig-nificantly faster reaction was observed (finished in 15 min) withGNT–AAO prepared at lower pH (smaller Au clusters). The influ-ence of size of gold structures on their catalytic properties is welldocumented and these results clearly show that increasing the sizeof gold structures significantly reduce catalytic activity of GNT–AAO membranes. In earlier reports, 4-NP reduction using metalnanoparticles was shown to be pseudo-first-order in the presenceof homogeneous or heterogeneous catalyst [8–11,31]. However, inthe present case, a linear relation of ln(Ct/C0) versus time is ob-served indicating that the reaction follows first-order kinetics(Fig. 6(b)) The slope of the line of the reaction using GNT–AAO withsmall clusters gives a first order rate constant k = 0.132 min�1

which reflects a relatively high catalytic reduction rate by compar-ison with other nano and micro gold nanoparticles synthesized bydifferent methods such as Frens method (0.006–0.036 min�1),

Fig. 4. (a)–(c) SEM images of GNT structures with smaller gold clusters with scheme showing the formation of these structures as result of pH (pH 8) influenced nucleation.(d)–(f) SEM images of GNT structures with larger gold clusters formed at higher pH (>9.5).

Fig. 5. Time dependent absorption spectra for the catalytic reduction of 4-nitrophenol (4-NP) (3.91 � 10�6 M) by NaBH4 (3.91 � 10�3 M) with GNT–AAO membrane presentas a catalyst. Scheme of reaction is shown on the right.

Fig. 6. (a) Concentration C (lmol L�1) versus time and (b) the linear relation of ln(Ct/C0) versus time graphs for 4-nitrophenol reduction (4-NP) using GNT–AAO membrane ascatalyst with different size of gold nanoclusters.

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136 Y. Yu et al. / Microporous and Mesoporous Materials 153 (2012) 131–136

conductive polymer – gold composite (0.037 min�1) and diatomsilica template (0.23 min�1) [7,8,11,30]. The GNT–AAO membraneswith large gold clusters inside nanotubes showed a significantlylower kinetic rate of 0.043 min�1 confirming the importance ofthe size of gold clusters for catalytic efficacy. When we doubled,tripled and quadrupled the dosage of GNT–AAO membranes inthe solution, the rate constant k correspondingly doubled, tripledand quadrupled. Although in surface catalysis zero-order reactionsare common on planar substrates, this change in reaction orderindicates that the reduction occurs via different rate-determiningsteps induced by tubular morphology of substrate. Further studiesare required to determine other critical parameters that govern thecatalytic process of GNT–AAO membranes. The influence of diffu-sion limitation on the rate constant was insignificant as the reac-tion mixture was stirred at a moderately high agitation rate(�500 rpm). This result indicates that the experiment can be scaledup for a more efficient, fast catalytic reduction reaction. The highcatalytic rate constant (k = 0.132 min�1) of GNT–AAO membranescan be explained by an accessible, high active surface area of nano-tubes and their nanostructured topography. It is well known thatcatalytic activity of gold nanoparticles is size dependent with sig-nificant increase of activity when particle’s size is reduced to<5 nm [3].

Finally, to check the reusability of GNT–AAO membranes, themembranes were cleaned after each reduction process of 4-NPwith Milli-Q water. The catalytic activity did not show a noticeabledecrease after several experiments indicating that the gold coatedmembranes could be recycled and reused as a stable catalystrepeatedly. The catalytic properties of GNT–AAO membranes canbe improved by optimization further reducing the size of goldnanoclusters on GNT. To achieve this goal, the gold depositionmethod should be adapted by changing the plating conditions(pH, temperature, concentration) or including specific additives[32]. The doping of GNT by other metals including Pd and Pt mightalso be considered as another promising strategy. Nitrophenolsand their derivatives involved in production of pesticides, herbi-cides, insecticides, and synthetic dyes increases the water pollutionby phenol and phenolic compounds. Even though the reaction ofcatalytic conversion of 4-nitrophenol was used as model, this studyis also technologically important for the development of methodol-ogies for reducing water pollution caused by phenolic compounds.

4. Conclusion

The fabrication of GNT–AAO membranes with distinct 2-Dnanotubular morphologies and catalytic properties was demon-strated. A simple gold electroless deposition into AAO membranesas template was used for fabrication process. The results showedthat catalytic properties of prepared GNT–AAO membranes wereinfluenced by the size of nanoscale gold structures inside nano-tubes created during electroless gold deposition process. Due tothe nanoclustered morphology, an excellent catalytic activity (cat-alytic rate constant k = 0.132 min�1) of GNT membranes was dem-onstrated for the reduction of 4-NP into 4-AP in the presence ofNaBH4 as reductant. The correlation between the size of goldnanoclusters and catalytic activity of GNT membranes was con-firmed showing the capability of controlling and further improving

their catalytic properties. The recycling of the GNT membrane cat-alyst was verified showing no deactivation effect after cleaning.This synthetic approach is generic and flexible and it is envisagedthat it can be applied for the preparation of a wide range of differ-ent metals (Pt, Pd, Ag, Ni, etc.) and different porous materials.

Acknowledgments

This work is supported by the Australian Research Council (DP0770930 and LP 098922), and the University of South Australiaand the Australian Microscopy and Microanalysis Research Facility(AMMRF). The authors would like to thank Industry partner Inp-haze Pvt. Ltd., Sydney for their support and Adelaide Microscopyfor their help with SEM characterization. Thanks to Leonora Vell-eman from Flinders University for her help during GNMsfabrication.

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