Efficient Charge Transfer in Au/CdSe Janus Nanoparticles Boosts Photocatalytic Hydrogen Production

Abstract

Metal–semiconductor heterostructures can outperform their individual components when their morphology and interfacial quality are finely tuned. Here we report a one‑step, aqueous synthesis of Au/CdSe Janus nanospheres that feature two hemispherical domains separated by a flat, crystalline interface. By adjusting the pH of the growth solution, we can produce a range of morphologies—from Janus spheres to heterodimers and multi‑headed structures—each with distinct interfacial characteristics. The Janus nanospheres exhibit a hydrogen evolution rate of 105.2 µmol h⁻¹ g⁻¹ under visible‑light irradiation, 3.9 times higher than the heterodimer control, due to superior charge‑transfer efficiency across the Au/CdSe interface. This work highlights the critical role of interface engineering in designing photocatalytic nanohybrids.

Introduction

Colloidal metal–semiconductor heterostructures have attracted significant attention for their unique optical and catalytic properties, which are crucial for solar‑to‑chemical energy conversion, photocatalysis, photoelectric devices, and photothermal therapy (e.g., refs [1–15]). In particular, plasmonic metal nanocrystals combined with chalcogenide semiconductors (CdX, Ag₂X, Cu₂X, PbX) can harvest visible light and transfer hot electrons to adjacent semiconductors, enhancing photocatalytic hydrogen evolution (refs [16–39]). However, the performance of such hybrids is strongly dependent on morphology, size, hybrid configuration, and, critically, the quality of the metal–semiconductor interface (refs [40–45]). Achieving a low‑energy, atomically flat interface is challenging due to lattice mismatch, yet it is essential for efficient charge separation and transfer.

In this study, we develop a controllable synthesis of water‑dispersed Au/CdSe Janus nanospheres with a high‑quality interface. By varying the reaction pH, we systematically investigate how interfacial properties affect photocatalytic hydrogen generation, providing insight for future heterostructure design.

Methods/Experimental

Materials

All reagents were purchased from commercial suppliers (Sinopharm, Shanghai; Amresco, USA) and used as received. Key chemicals include HAuCl₄·4H₂O (99.99 %), AgNO₃ (99.8 %), glycine (99.5 %), Se powder (99.5 %), L‑ascorbic acid (99.7 %), NaOH (96 %), Cd(NO₃)₂·4H₂O (99 %), HCl (36–38 %), HMT (99 %), NaBH₄ (96 %), and CTAB (99 %).

Synthesis of Au Nanoparticles

Au seeds (4.5 mL) were prepared by mixing 500 µL of 5 mM HAuCl₄ and 5 mL of 0.2 mM CTAB, followed by the addition of 600 µL of ice‑cooled 10 mM NaBH₄. After 2 h, 120 µL of this seed solution was added to a growth mixture (190 mL H₂O, 4 mL 10 mM HAuCl₄, 9.75 mL 0.1 M CTAB, 15 mL 100 mM ascorbic acid) and stirred gently overnight at room temperature to yield ~22 nm Au spheres.

Synthesis of Au–Ag Bimetallic Nanoparticles

Au nanoparticles (8.0 nM) were mixed with 5 mL of 200 mM glycine and adjusted to pH = 2.5, 4.5, 7.2, or 8.1 by dropwise HCl or NaOH. After 1 min at 30 °C, 15 µL of 100 mM AgNO₃ was injected and the mixture stirred for 10 h at 30 °C. The resulting Au–Ag seeds were used directly for CdSe growth.

Synthesis of Au/CdSe Janus Heterostructures

2 mL of Au–Ag seeds, 6 mg Se powder, 0.01 mL 100 mM Cd(NO₃)₂, and 40 µL 10 mM NaBH₄ were mixed and stirred vigorously at 90 °C for 2 h. The product was centrifuged (9500 rpm, 5 min) and washed twice with water. By changing the pH of the Au–Ag synthesis step, Janus, heterodimer, double‑headed, or multi‑headed nanostructures were obtained.

Photocatalytic Activity Measurement

Hydrogen evolution was measured in a quartz tube reactor. 100 mg of photocatalyst was dispersed in 50 mL of aqueous solution containing 5 mL lactic acid (sacrificial agent). After purging with air for 30 min, the suspension was illuminated with a 300‑W Xe lamp (λ > 420 nm) while maintaining 6 °C with a water‑cooling bath. Hydrogen was quantified by online GC (Tianmei GC‑7806).

Characterization

TEM (JEOL 2010 HT, 200 kV) and HRTEM (JEOL 2010 FET, 200 kV) were used to analyze morphology and interfaces. UV‑Vis spectra were recorded with a TU‑1810 and Cary 5000 spectrometer at room temperature.

Results and Discussion

The synthesis route is illustrated in Figure 1. After CTAB‑stabilized Au spheres were prepared, a thin Ag wetting layer was deposited at low pH (2.5). Subsequent selenization and Cd²⁺ exchange yielded a sharp, flat Au/CdSe interface. TEM images (Figure 2) show the evolution from Janus nanospheres (2 h) to larger CdSe shells (3 h). HRTEM confirms lattice spacings of 0.20 nm (Au fcc (200)) and 0.21 nm (CdSe (220)), while EDX mapping (Figure 4) verifies the presence of Au, Cd, Se, and residual Ag.

Figure 5 demonstrates that the pH of the Ag deposition step determines the final morphology: pH = 2.5 gives Janus spheres; 4.5 produces heterodimers; 7.2 yields symmetric double‑headed particles; 8.1 leads to multi‑headed structures. Lower pH favors slower CdSe growth and a flatter interface, reducing interfacial strain.

Optical spectra (Figure 6) reveal a blue‑shift of the Au plasmon band upon Ag coating (from 522 nm to 500 nm) and a red‑shift after CdSe growth, reflecting increased effective refractive index. Higher CdSe loading also broadens the extinction band due to inhomogeneous shell thickness.

Photocatalytic hydrogen evolution (Figure 7) shows a clear trend: multi‑headed <0.16 µmol h⁻¹ g⁻¹, double‑headed 21.4 µmol h⁻¹ g⁻¹, heterodimers 26.7 µmol h⁻¹ g⁻¹, and Janus nanospheres 105.2 µmol h⁻¹ g⁻¹. The superior performance of Janus structures is attributed to their flat, low‑strain interface, which facilitates rapid electron transfer from CdSe to Au and suppresses recombination (illustrated in Figure 8).

Conclusion

We have demonstrated a facile, aqueous route to synthesize Au/CdSe Janus nanospheres with a flat, high‑quality interface by controlling the pH of the Ag deposition step. The Janus nanospheres exhibit a 3.9‑fold higher hydrogen evolution rate compared to heterodimer counterparts, underscoring the importance of interface engineering for efficient photocatalysis.

Availability of Data and Materials

All data generated or analyzed during this study are included in this article and its supplementary information file.