Date Published: December 25, 2014
Publisher: John Wiley and Sons Inc.
Author(s): Paul G. O’Brien, Amit Sandhel, Thomas E. Wood, Abdinoor A. Jelle, Laura B. Hoch, Doug D. Perovic, Charles A. Mims, Geoffrey A. Ozin.
Gaseous CO2 is transformed photochemically and thermochemically in the presence of H2 to CH4 at millimole per hour per gram of catalyst conversion rates, using visible and near‐infrared photons. The catalyst used to drive this reaction comprises black silicon nanowire supported ruthenium. These results represent a step towards engineering broadband solar fuels tandem photothermal reactors that enable a three‐step process involving i) CO2 capture, ii) gaseous water splitting into H2, and iii) reduction of gaseous CO2 by H2.
Catalyst Fabrication: Silicon nanowires were fabricated using a metal‐assisted chemical etching (MaCE) technique. p‐type silicon wafers were cut into 1 inch squares and then cleaned with ethanol, acetone and de‐ionized water. The wafers were further cleaned with piranha solution (H2SO4:H2O2 = 3:1 by volume) for 3 h and then rinsed with de‐ionized water. Subsequently, the wafers were immersed in an etching solution consisting of 5 M HF, 0.02 M AgNO3, and 3 mL of 10% HF solution in order to remove surface oxides. The solution is then placed in an autoclave and allowed to etch for 1 h at room temperature. After the etching process, silver dendrites covering the silicon nanowires were washed off with deionized water. To ensure all the silver nanoparticles and dendrites were removed the etched wafers were placed in concentrated nitric acid (18 M HNO3) for 30 min. The etched wafers were then washed and dried before being cut into 1cm2 pieces. Eagle XG and P‐type polished silicon wafers were cleaned in a solution of sulfuric acid/hydrogen peroxide (3:1 v/v) and then rinsed with distilled water. Ru was sputtered onto these samples which were subsequently cut into 1 cm2 squares. The deposition was carried out in a custom‐built sputtering system (Kurt J. Lesker Co.) by radio frequency (RF) magnetron sputtering using a 99.95% pure Ru sputtering target purchased from Angstrom Sciences, Inc. The base pressure of the sputtering chamber was pumped down to 1 × 10−7 Torr before argon was introduced into the chamber at a flow rate of 20 sccm. The chamber pressure was set to 3 mTorr during the deposition, which was carried out at room temperature. The forward power was 100 W and the substrate‐to‐target distance was 14 cm. The sputtering process was terminated when 10 nm of Ru, as measured from an in‐situ thickness monitor (SQM‐242 from Sigma), had been deposited.