Ultima 4 system

A typical growth solution of the AuNPs was prepared as follows: 3-mL of the aqueous solutions tryptophan (6.25 mM) was added in different glass bottles containing diluted NaOH (5 mM). This solution was brought to 95 °C and the synthesis of the AuNPs was initiated by adding 1.0 mL of 20 mM HAuCl₄. The AuNPs synthesis reaction was allowed to run for 48 hours at 95 °C. Centrifugation of the AuNPs solution was carried out at 10,000 rpm for 15 min to remove unbound tryptophan molecules and NaOH used in the synthesis. UV-Vis spectrophotometer (Optizen-2120) was used to record UV-Vis spectrum of the AuNPs in the wavelength range from 400 to 900 nm, with a resolution of 5 nm. Transmission electron microscope (TEM) images of the AuNPs were obtained from a Hitachi HF-3300. TEM samples were prepared by adding 100 μL of the AuNPs solution onto the 200-mesh Formvar-coated copper grid. X-ray diffraction (XRD) spectrum of the of the AuNPs thin film was measured by using Rigaku Ultima-IV System by operating at voltage 40 kV and current of 30 mA using Cu K radiation. FT-IR spectra of the centrifuged AuNPs was collected by using an infrared spectrometer (Thermo Electron Nicolet-6700).

The morphologies of the samples were investigated by field‐emission SEM (FESEM, ZEISS SIGMA 500) and TEM (FEI Tecnai G20). Crystal structures of the samples were conducted by XRD using a Rigaku Ultima IV system with Cu Κα radiation. The surface chemistry was analyzed by XPS spectra using an AXIS Supra facility while the ISI‐XPS spectra were detected under UV light irradiation at 365 nm. The UPS spectra were recorded with a He I radiation (hν = 21.22 eV) source. A platinum wire and Ag/AgCl were employed as the counter electrode and reference electrode, respectively. The electrolyte was a 1 mol L⁻¹ Na₂SO₄ aqueous solution and the Mott–Schottky test frequency was 1000 Hz.