Symmetry-breaking synthesis of Janus Au/CeO2 nanostructures for visible-light nitrogen photofixation

Precise manipulation of the reactive site spatial distribution in plasmonic metal/semiconductor photocatalysts is crucial to their photocatalytic performance, but the construction of Janus nanostructures through symmetry-breaking synthesis remains a significant challenge. Here we demonstrate a synthetic strategy for the selective growth of a CeO2 semi-shell on Au nanospheres (NSs) to fabricate Janus Au NS/CeO2 nanostructures with the assistance of a SiO2 hard template and autoredox reaction between Ag+ ions and a ceria precursor. The obtained Janus nanostructures possess a spatially separated architecture and exhibit excellent photocatalytic performance toward N2 photofixation under visible-light illumination. In this scenario, N2 molecules are reduced by hot electrons on the CeO2 semi-shell, while hole scavengers are consumed by hot holes on the exposed Au NS surface, greatly promoting the charge carrier separation. Moreover, the exposed Au NS surface in the Janus structures offers an additional opportunity for the fabrication of ternary Janus noble metal/Au NS/CeO2 nanostructures. This work highlights the genuine superiority of the spatially separated nanoarchitectures in the photocatalytic reaction, offering instructive guidance for the design and construction of novel plasmonic photocatalysts.

Growth of the Au NSs. The monodisperse Au NSs were prepared using a seeded growth method as described in previous work. 1 The whole process can be divided into three steps: (1) growth of the small Au NSs; (2) growth of the large Au nanopolyhedrons; and (3) growth of the large Au NSs.
Growth of the small Au NSs: The small Au NSs were prepared using a seeded growth method. Specifically, the seed solution was made by injecting a freshly prepared, ice-cold NaBH 4 solution (10 mM, 600 L) into an mixture solution composed of CTAB (0.1 M, 9.75 mL) and HAuCl 4 (10 mM, 250 L) under vigorous stirring.
The resultant seed solution was kept under gentle stirring for 3 h before use. The growth solution was made by the sequential addition of CTAB (0.1 M, 9.75 mL), HAuCl 4 (10 mM, 4 mL), and AA (0.1 M, 15 mL) into DI water (190 mL), followed by the injection of the as-prepared seed solution (0.12 mL). The resultant solution was mixed by gently inversion and kept undisturbed overnigh at room temperature. The obtained small Au NSs were concentrated by centrifugation and redispersion into DI water (55 mL) for further use.

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Growth of the large Au nanopolyhedrons: The small Au NSs were employed as seeds for the growth of the large Au nanopolyhedrons. Typically, the small Au NSs (4 mL) was first added into a CTAC solution (25 mM, 30 mL), followed by the sequential addition of AA (0.1 M, 0.75 mL) and HAuCl 4 (10 mM, 1.5 mL). The resultant solution was placed in an air-bath shaken (45 C, 160 rpm/min) for 3 h. The as-prepared large Au nanopolyhedron sample was centrifuged and redispersed into CTAB (20 mM, 30 mL) for further use. Growth of the CeO 2 nanocrystals. The preparation of CeO 2 nanocrystals was similar to that of the core@shell nanostructures except that the Au NS solution was replaced by DI water and the volumes of DI water (9.55 mL), AgNO 3 (0.2 mL) and Ce(AC) 3 (0.2 mL) were changed to keep the overall volume to 10 mL.
Photocatalytic N 2 fixation under the visible-light irradiation. The photocatalytic N 2 fixation reaction was carried out in a customized reactor (diameter = 3 cm) with three ends. Two side ends were used as inlet and outlet for gas flow, while the middle end was equipped with a quartz window on the top for light illumination.
Typically, the photocatalyst (0.7 mg) was dispersed into DI water (8 mL). CH 3 OH (2 mL) was added as the hole scavenger. High-purity N 2 was bubbled in the mixture solution at a speed of 20 mLmin -1 for 10 min before each photocatalytic reaction. A continuous Xe lamp (CEL-HXF 300 W) equipped with an AM 1.5 G and a 420−780 nm filters was employed as the visible light source. The optical power density was 300 S-5 mWcm -2 . A circulation cooling system was used to keep the reaction solution temperature at 25 C. The N 2 photofixation reaction was performed under visible-light illumination for 2 h. N 2 was bubbled in the reaction solution at a speed of 20 mLmin -1 for the entire photocatalytic process. The produced NH 3 in the supernatant was determined using indophenol-blue method. 3,4 Each photocatalytic experiment was repeated three times.
The 15 N 2 isotope labelling experiments were conducted in a customized gastight reactor (diameter = 3 cm) with three ends. Two side ends were used as inlet and outlet for gas flow, while the middle end was equipped with a quartz window on the top for light illumination. The two side ends were closed with glass stopcocks during photocatalytic reaction. Before the light illumination, high-purity Ar was firstly bubbled in the reaction solution at a speed of 100 mLmin -1 for 30 min, followed by the 15 N 2 at a speed of 20 mLmin -1 for 15 min. Characterization. TEM imaging was conducted on an HT7700 electron microscope operated at 100 kV. The sizes and thicknesses were measured from the TEM images, with more than 300 particles being counted.
HRTEM, HAADF-STEM imaging and EDX elemental mapping were performed on an FEI Talos F200S microscopy. The extinction spectra were measured with a Hitachi U-3900 ultraviolet/visible/NIR spectrophotometer. XPS spectra were obtained on a Thermo Scientific ESCALAB 250Xi spectrometer. XRD patterns were performed on a Smart Lab Se diffractometer equipped with Cu K radiation. Inductively coupled plasma optical emission spectroscopy (ICP-OES) was carrier out on a PerkinElmer Optima 7300 DV system. 1 H NMR spectra were measured on a Bruker Avance Ⅱ 400 NMR spectrometer.

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Supplementary Figures   Fig. S1 HRTEM images of a single ternary Janus Au NS/SiO 2 /CeO 2 nanostructure and three selected areas.