Elsevier

Journal of Catalysis

Volume 377, September 2019, Pages 389-399
Journal of Catalysis

Catalysis of Au nano-pyramids formed across the surfaces of ordered Au nano-ring arrays

https://doi.org/10.1016/j.jcat.2019.07.038Get rights and content

Highlights

  • Ordered array of nano-rings is a seldom reported morphology in literatures.

  • The coverage of arrays is ∼100% over 5 × 5 mm2 substrate (up-scaling possible).

  • Atomic scale active sites are directly observed by advanced electron microscopy.

  • Au nano-ring array with active sites demonstrates extraordinary catalytic activity.

  • Use of low-hazard materials is helpful in scaled production and mass application.

Abstract

Gold (Au) is a noble metal that has been widely researched due to its high catalytic performance-to-cost ratio. The catalysis could be improved by fabrication of nanostructured arrays on Au thin film with a large area. In the present paper, a facile NSL-RIE approach comprising of nanosphere lithography followed by oxygen plasma reactive ion etching has been developed to fabricate ordered Au nano-ring arrays with almost 100% substrate coverage. The preferential etching of {1 1 1} lattice planes of Au gives rise to the formation of a number of Au nano-pyramids across the entire surface of the nano-ring arrays. The sharp tips along with the Au atoms around the edges of these nano-pyramids provide active sites that perform strong catalysis. Meanwhile, Moiré fringes are observed over the etched Au surface which is caused by the rotation of the partial Au lattice with respect to the original Au lattice. This lattice distortion provides high surface energy allowing the significant improvement of the catalysis of Au nano-ring arrays. The direct evidence is proven by the enhanced electrochemical properties of the as-produced Au nano-ring array, which has high efficiency in ethanol oxidation reactions (EORs) with low energy input requirement. Electrochemical testing with varying physical parameters shows that the detecting limit of ethanol is ∼5.0 mM and strong signals come with high concentrations of mobile charge carriers at extreme pH values. Long continuous CV scans reveal that in alkaline medium, C1 pathway could become increasingly preferential over C2 pathway, and thus making the Au nano-ring array structure useful in the field of direct ethanol fuel cell.

Introduction

In view of the fast depletion of fossil fuels and environmental pollution from their combustion, sourcing for alternative and renewable clean energies has been an urgent need for sustainable development. Ethanol is cheap, safe, abundantly available, and could be applied across many fields, thus becoming an excellent choice of substituent to petrol and natural gas. As such, the research on ethanol oxidation reactions (EORs) is important; lower energy input requirement and more complete oxidation would become significant progresses, and these usually require catalysis by an electroactive material [1], [2].

Most of the commercial EOR applications are either having the participation of strong oxidative chemicals like acidified potassium dichromate (VI) solution, or catalysed by transition metals like copper (Cu). The main issues on the former are the associated pollution and environmental unfriendliness; while the problems with the latter arise from the partial catalysis and incomplete oxidation, which could release more toxic products like ethanal. Hence, current research on EOR electrocatalysts is focused on noble metals; gold (Au) is exceptionally popular as it has high performance-to-cost ratio compared to platinum-group (Pt, Pd, Ru, etc.) and silver (Ag) [1], [2], [3], [4], [5], [6], [7].

To fabricate novel Au electrocatalysts, the efficiency of catalysis could be enhanced by making nanostructured arrays on electrode surfaces. Fabrication of arrays could be accomplished by many ways; lithography [8], [9], [10], [11], [12], [13] is a good choice due to its capability to scale up compared to focused ion beam (FIB) patterning [14], [15], [16], [17], [18], [19] and low chemical hazard compared to wet chemical etching [20], [21], [22], [23], [24]. Some common lithographic techniques include nanoimprint lithography (NIL) [25], [26], [27], [28], photolithography (PhL) [29], [30], [31], [32], [33], [34], electron beam lithography (EBL) [35], [36], [37], [38], [39], nanosphere lithography (NSL) [11], [12], [13], [27], [40] and scanning probe lithography (SPL) [27], [41], [42], [43]. These approaches are listed and compared in Table 1.

Array structures have a wide spectrum of applications, including but not limited to light-emitting diode devices [12], [13], field emitters [12], [13], solar cells [44], [45], [46], electro-/photo-catalysis [13], [47], anti-reflection coatings [12], [13], surface property modifications [12], [13], catalysed reactions [13], [48], [49], Raman spectroscopy [50], and even biomedical applications [51]. However, the most reported application of array structures is electro-/photo-catalysis, mainly owing to their largely increased specific surface area and number of active sites, as well as reduced activation energy along with highly ordered structure. Based on these, many fundamental studies have been made through modelling and simulation, and they indeed proved that active sites are one of the major factors that boost the electrochemical performance in catalysis.

In contrast to most of the research works in array fabrication, this research intends to produce ordered Au nanostructured arrays in large area. Therefore in this work, we introduce a facile yet simple method that combines NSL with reactive ion etching (RIE), which could assemble ordered Au nano-ring array in large area with almost 100% substrate coverage. Apart from large area fabrication of arrays, observation of the active sites is seldom reported although these sites are believed to locate at the corners and/or edges of Au [54]. Thus, looking for the direct evidence of active sites through advanced electron microscopy is another key intention of this work.

The electrochemical tests reveal enhanced catalytic activities of the array structure compared to planar Au thin film without array structure. Even though EOR may occur randomly via C1 (producing CO2 as final product) or C2 (producing acetic acid or acetates as final products) pathway [55], the Au thin films with ordered nano-ring array structure show increased tendency in preferential working via C1 pathway as time passes, and this is in agreement with the initiative to utilise ethanol as a source of energy in the direct ethanol fuel cells [56]. Last but not least, every step in this work uses low-hazard materials (including the active materials, the templates and the gaseous etchants), and this will be of great help in scaled production and potential mass application of the products.

Section snippets

Fabrication of ordered Au nano-ring arrays

P-type Si 〈1 0 0〉 wafers were treated in piranha solution and rinsed with ethanol before drying. Au thin film with the thickness of 100 nm was deposited by DC magnetron sputtering. Monodispersed commercial polystyrene (PS) colloidal spheres were self-assembled on Au thin film via the vapour-liquid-solid (VLS) method, which formed a close-packed monolayer. The samples were then etched by a reactive ion etcher (ES371, Nippon Scientific Co. Ltd., Japan) to obtain the Au nano-ring array structure.

Morphology and crystallography of ordered Au nano-ring arrays

The crystalline phases of the samples were confirmed by thin film XRD. Fig. 1 shows the XRD patterns of the Si/Au/PS sample before (black) and after (red) RIE where most of the visible diffraction lines could be ascribed to the face-centred cubic (FCC) pristine Au (JCPDS #04-0784). The diffraction lines at 2θ ≈ 8.0° and 2θ ≈ 56.4° are contributed by the self-assembled PS nanospheres before the RIE process, as verified by the XRD pattern of the Si/PS sample, and they disappear after the RIE

Conclusions

This research has shown that the facile NSL-RIE approach is a fast and effective way to fabricate ordered Au nanostructured arrays on Si wafers; compared to other methods, this approach is much safer and easier. Under the optimised RIE parameters, the coverage of the Au nano-rings is almost 100% except intrinsic defective regions prior to the RIE process, and with different etching durations, experiments have revealed that the height of Au nano-rings has a reduction rate of ∼2.31 nm/min.

Acknowledgements

This research was support by the MOE AcRF Tier 1 grant M4011528. The XRD analyses were performed at Facility for Analysis, Characterisation, Testing and Simulation (FACTS) Lab; the FEG-SEM/FIB characterisations were carried out at Microelectronics Reliability and Characterisation (MRC) Lab and reactive ion etching was done in the Microelectronics Failure Analysis (MFA) Lab. Special thanks to Mr Tan Jok Boon Eric, the technical staff of MRC and MFA labs in MSE NTU, for his assistance in carrying

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