Anisotropic dual-plasmonic hetero-nanostructures with tunable plasmonic coupling effects

The influence of plasmonic coupling effects between different components in Au NRs@Cu2−xSe nanostructures on their characteristics was studied. To this aim, anisotropic Au@Cu2−xSe hetero-nanostructures with well-controlled design and optical properties were obtained. The LSPR bands of gold and copper selenide are superpositioned in the NIR region, resulting in superior photocatalytic properties of the nanostructures.

added, while stirring at 700 rpm. The mixture was stirred for 90 minutes, and the disappearance of yellow color was observed. The pH value was adjusted by addition of 0.21 mL (Au NRs-1 with aspect ratio 2.4) or 0.3 mL (Au NRs-2 with aspect ratio 3.8) of concentrated HCl and stirring at 400 rpm for 15 minutes. Next, 0.125 mL of 0.064 M ascorbic acid was injected, while the mixture was vigorously stirred at 1200 rpm. After 30 s, a 0.08 mL of seed solution was introduced into the growth mixture, while maintaining vigorous stirring. The mixture was stirred for 30 s and left undisturbed at 30°C overnight. Obtained Au NRs were washed by centrifugation at 7000 rpm for 30 min. After the removal of the supernatant, product was redispersed in 10 mL of 0.1 M hexadecyltrimethylammonium chloride (CTAC).

Synthesis of Au NRs@Cu 2-x Se
Au NRs-1 with aspect ratio 2.4: 0.25 mL as prepared Au nanorods were diluted with 1.25 mL of water and mixed with 0.5 mL of 0.02 M CTAC and 0.2 mL of 0.1 M L-ascorbic acid (AA) aqueous solutions at 40 °C. While stirring at 1200 rpm, 10, 30, 50, 70, or 100 µL of 0.01 M SeO 2 aqueous solution was added dropwise utilizing a syringe pump with an injection rate of 10 µL/min. The reaction was allowed to proceed for 10 mins after the injection had been finished. Then, Au@Se dispersion was washed twice with water to remove any free Se NPs and redispersed in 2 mL of 5 mM CTAC. 15 µL of 0.2 M CuSO 4 and 0.2 mL of 0.1 M AA aqueous solutions were added into the colloid simultaneously at room temperature. The mixture was left under vigorous stirring at 1200 rpm for 40-60 mins (time increases for thicker Se shell). The resulting products were purified through centrifugation at 4000 rpm for 5 mins and redispersed in water.
Au NRs-2 with aspect ratio 3.8: followed the same protocol, except volumes of 0.01 M SeO 2 aqueous solution used were 30, 50, 70, 100 µL and centrifugation was performed at 7000 rpm for

Evaluation of the photocatalytic performance
The photocatalytic activity of the Au NRs@Cu 2-x Se nanostructures was evaluated by the photodegradation of Rhodamine B (RhB) under a Xe lamp irradiation equipped with an ultravioletcut (λ > 420 nm) filter at 24 °C. Photocatalytic reactions were conducted in a cylindrical quartz reactor equipped with the outer shell for water circulation. Concentration of gold in colloidal solutions was adjusted to 214.14 mg/L based on results of ICP-OES analysis. 5 mmol/L solution of RhB was added to 0.25 mL of Au NRs@Cu 2-x Se dispersion to get a total volume of 2 mL. Prior to illumination, the mixture was magnetically stirred in the dark for 1 h to allow for RhB molecules adsorption onto the catalyst surface. At irradiation time intervals of 30 min, dispersion was collected and centrifuged to remove the photocatalyst particles. After the catalyst was separated, the ultraviolet spectrum of the supernatant was recorded. Then, nanocatalysts were redispersed in the supernatant and illuminated again. During the photodegradation process, the RhB concentration was monitored by measuring the absorbance value at 553 nm.

Characterization and Instrumentation
Transmission electron microscopy (TEM and HRTEM) images were obtained on an FEI Titan 80-300 analytical transmission electron microscope and a JEOL JEM-1400Flash microscope operating at 120 kV. The TEM samples were prepared by dropping colloidal solutions of nanoparticles onto the surface of 300-mesh carbon-coated copper grids. The optical extinction spectra of colloidal nanoparticles were recorded at room temperature on a Shimadzu UV-2600 Plus spectrophotometer equipped with an integrating sphere in plastic cuvettes of 1-cm optical path length. ICP-OES analysis was performed utilizing SpectroBlue-FMT36 ICP OES spectrometer.