Silicon nanowires fabricated by porous gold thin film assisted chemical etching and their photoelectrochemical properties Lifeng Liu n , Xiao-Qing Bao International Iberian Nanotechnology Laboratory (INL), Av. Mestre Jose Veiga, 4715-330 Braga, Portugal article info Article history: Received 16 January 2014 Accepted 23 March 2014 Available online 29 March 2014 Keywords: Silicon nanostructures Metal assisted etching Nanoporous gold Solar hydrogen evolution Photoelectrochemistry abstract Nanoporous gold thin films fabricated by dealloying commercially available 12 kt gold leaf have been utilized as masks to catalyze the formation of silicon nanowires from p-type silicon wafers in a HF/H 2 O 2 mixture. The resulting silicon nanowires are found to be 40–100 nm in diameter and highly porous with an average pore diameter of ca. 4 nm. These nanowires are used as photocathodes for solar hydrogen evolution, and are found to exhibit a photocurrent as high as 6.05 mA cm À2 at a potential of 0 V versus reversible hydrogen electrode (RHE), which is 430 times higher than that of the planar silicon of the same type. The electrochemical impedance spectroscopy result shows that the charge transfer resistance of the nanowire photocathodes is much smaller than that of the planar silicon, revealing a favorable charge transfer kinetics. & 2014 Elsevier B.V. All rights reserved. 1. Introduction Silicon nanowires (SiNWs) have been extensively investigated in the past two decades for their unique electronic, optical, catalytic, electrochemical and photoelectrochemical properties which can find many applications in nanoelectronics [1], biosen- sors [2], photocatalysis [3], lithium-ion batteries [4], solar cells [5] and photoelectrochemical cells [6–8]. Various methods have so far been used to fabricate SiNWs including chemical vapor deposition [9,10], molecular beam epitaxy [11], electrodeposition [12], super- critical point drying [13] and metal assisted chemical etching (MACE) of silicon [14], among which the MACE has recently attracted tremendous interest because it is a very simple, low cost method allowing for the fabrication of high quality SiNWs with controlled dimensions and orientations [14]. Central to the MACE process is to pattern the Si surface with a porous mesh of noble metals such as Ag, Au or Pt, which usually can be achieved through optical lithography [15], nanosphere lithography [16], polymer lithography [17], laser interference lithography [18] or porous alumina masking [19]. These lithogra- phy and masking techniques involve complicated steps toward patterning and are usually time-consuming. Although the MACE can also be achieved in a simpler way using the mixture of hydrofluoric acid (HF) and noble metal salts (e.g. AgNO 3 ) as etchant [14], thus-obtained NWs generally have a rather wide size distribution and also a non-uniform diameter for individual NWs from one end to the other. In this work, we for the first time use the dealloyed nanoporous gold (NPG) thin film as both a mask and catalyst to fabricate SiNWs via MACE. The obtained NWs are porous, uniform in diameter and highly crystalline. Moreover, they exhibit excellent photoelectrochemical performance towards solar hydrogen evolu- tion, compared with planar silicon of the same type. 2. Materials and methods The AuAg alloy foils (12 ct, 50 wt% Au) were purchased from Wrights of Lymm Ltd., and have a nominal thickness of 100 nm. The dealloying was performed by floating the alloy foils onto concentrated nitric acid (70%, Sigma) in a petri dish for a certain period of time (typically 2–4 h). Afterwards, the nitric acid was carefully removed using a pipette, and the dealloyed foil was washed with deionized water with caution for three times before it was transferred onto a p-type Si substrate (B-doped, 8–15 Ω cm) which was dipped in a 2% HF solution for 5 min prior to use. The MACE was carried out in a mixture of HF and H 2 O 2 (v/v 12:1) at room temperature for 30 min. Afterwards, the wafer was thor- oughly rinsed followed by emerging into a gold etching solution (KI:I 2 :H 2 O ¼ 4 g:1 g:40 ml) to remove the NPG masks. The samples were characterized by scanning electron micro- scopy (SEM, FEI Quanta 650) and transmission electron micro- scopy (TEM, FEI Titan ChemSTEM80-200, probe corrected). The reflectivity was measured using a UV–vis spectrometer (Shimadzu Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/matlet Materials Letters http://dx.doi.org/10.1016/j.matlet.2014.03.145 0167-577X/& 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ351 253 140112; fax: þ351 253 140119. E-mail address: lifeng.liu@inl.int (L. Liu). Materials Letters 125 (2014) 28–31