Au@Cu₇S₄ yolk–shell nanoparticles as a 980 nm laser-driven photothermal agent with a heat conversion efficiency of 63%

Abstract

We report the preparation of Au@Cu₇S₄ yolk–shell nanoparticles (NPs) via an inward replacement strategy based on the Kirkendall Effect. Due to the surface plasmon resonance (SPR) effect enhanced absorption in the near-infrared (NIR) region and the unique yolk–shell structure, the Au@Cu₇S₄ NPs exhibit excellent photothermal performance with a heat conversion efficiency up to 63% under 980 nm laser irradiation.

---

The photothermal effect of nanostructures as an emerging research spot has received increasing attention due to the promising applications in water splitting,¹ organic degradation,² CO₂ reduction,³ Suzuki coupling reactions,⁴ and especially in cancer photothermal therapy.^5–8 Generally, the photothermal effect can be described as the conversion of heat from photon energy, which involves the excitation of the surface plasmon band of nanostructures and the subsequent fast thermal relaxation of the exciton.^9,10 Therefore, nanostructures with unique surface plasmon resonance (SPR) absorption properties have been widely explored as photothermal agents, such as Au based nanostructures (nano-rods,¹¹ -cages,^12,13 and -stars¹⁴), Pd-based nanostructures,¹⁵ carbon-based materials (carbon nanotubes¹⁶ and graphene¹⁷), copper sulfides (Cu_xS, 1 ≤ x ≤ 2)^5,7,18–20 etc. Among them, due to their low cost, high stability, low cytotoxicity, and the intrinsic SPR induced NIR absorption properties, Cu_xS are becoming one class of most popular photothermal materials in recent years.^5–7,20,21 Different Cu_xS (like flower-like CuS superstructures,²⁰ hollow CuS nanoparticles (NPs),⁶ and Cu₉S₅ nanocrystals⁵) or Cu_xS based composites (like Fe₃O₄@ Cu_xS core–shell NPs,⁷ NaYbF₄: 2%Er³⁺/20%Gd³⁺@SiO₂–NH₂–CuS,¹⁹ and Au–Cu₉S₅ hybrid NPs¹⁸) in various morphologies and structures were developed and investigated as photothermal agents. Despite these successes, the Cu_xS still suffer from relatively low photothermal conversion efficiency. The exploration of efficient Cu_xS photothermal agents remains great challenges.

Recently, Jiang et al. demonstrated that, as a result of coupling effect of SPR originating from the collective electron and hole oscillations, the combination of Au NPs with Cu_xS could significantly enhance SPR of Cu_xS and consequently improve the photothermal performance.¹⁸ On the other hand, we also noticed that the unique yolk–shell structure can give rise to the multiple reflections of light between cores and shells within the interior voids, beneficial for the full utilization of light energy, which has been widely applied for the designing efficient optical functional materials.^22,23 Herein, inspired by these two phenomena, we carefully designed and synthesized Au@Cu₇S₄ yolk–shell NPs via an inward replacement strategy based on the Kirkendall Effect using Au@Cu₂O core–shell NPs as templates. As expected, the materials showed a powerful photothermal performance with the heat conversion efficiency up to 63% under a 980 nm laser irradiation. It should be noted that, apart from being considered as a strategy for enhancing the photothermal properties of Cu₇S₄ NPs, the construction of such unique Au-contained yolk–shell structure might make the materials promising platforms as drug deliverers.²³

As illustrated in Scheme 1, the preparation of Au@Cu₇S₄ yolk–shell nanoparticles is based on an inward replacement strategy using Au@Cu₂O core–shell nanoparticles as templates, which typically involves two steps.^24–26 Firstly, using Au nanoparticles as seeds and polyvinylpyrrolidone (PVP) as a structural directing agent, a polycrystalline Cu₂O layer was formed and coated outside the Au cores to get an Au@Cu₂O core–shell structure.²⁷ Secondly, under air atmosphere, Cu₂O shells would be converted into Cu₇S₄ shells immediately when suspended in the Na₂S solution because of the much smaller solubility product constant of Cu₇S₄ (K_sp ≈ 10⁻⁴⁸) than that of Cu₂O (eqn (1)).²⁵

14Cu₂O + 16S²⁻ + O₂ + 16H₂O → 4Cu₇S₄ + 32OH⁻ (1)

Scheme 1 Schematic illustration of the preparation process of the Au@Cu₇S₄ yolk–shell NPs.

Owing to the Kirkendall Effect induced net directional flow of materials at the template/reactant interface, the voids would be appeared in Cu₇S₄ shells accompanied by the increasing of particle diameter, consequently resulting in the formation of Au@Cu₇S₄ yolk–shell structures.^24,25

Au NPs were prepared by a standard citrate reduction procedure²⁸ and further observed by transmission electron microscopy (TEM). As shown in Fig. 1a and S1,† they exhibit uniform spherical morphology with high crystallinity and possess an average diameter of 20.3 ± 2.6 nm. Fig. 1b is the TEM image of Au@Cu₂O nanoparticles. It is apparent that they show typical core–shell structures and that each of them only contains one individual Au NP core. The high-resolution TEM (HRTEM) image in Fig. 1c and the Selected Area Electron Diffraction (SAED) pattern in Fig. S2† confirm the polycrystalline characteristics of the Cu₂O shells, which is well consistent with the previous report.²⁷ Specifically, the lattice spacings of 0.243 and 0.300 nm corresponds to the (111) and (110) planes of Cu₂O, respectively.²⁹ The morphology and interior structure of Au@Cu₇S₄ NPs are showed in Fig. 1d and e. Compared with Au@Cu₂O, obvious voids were formed in Au@Cu₇S₄ NPs, transferring the structure from core–shell to yolk–shell. This is believed to be the result of Kirkendall Effect.²⁴ Meanwhile, we also notice that many nanoplates were growing from the polycrystalline Cu₇S₄ shell. This result suggested that the structure reconstruction of Cu₇S₄ occurred during the replacement reaction. The particle size distribution histograms for Au@Cu₂O and Au@Cu₇S₄ are drawn in Fig. 1f and their average particle radii are determined from the mean value of Gaussian distribution to be 80.3 and 88.5 nm, respectively. This observed increase in diameters further confirms the formation of hollow interior inside the Cu₇S₄ shells. In addition, it should be noted that, apart from endowing such composite materials many attractive properties, like plasmonic effect, Au NPs also play critical roles in mediating the growth of Cu₂O in nanoscales. The pure Cu₂O particles were prepared in the absence of Au NPs as control experiments. As expected, they show a much larger particle size of around 200 nm (Fig. S3†). The similar conclusion was also obtained by others, who demonstrated that the diameter of Au@Cu₂O NPs could be tailored by controlling the adding amount of Au particle seeds.²⁷

Fig. 1 (a–e) TEM and HRTEM images of (a) Au, (b and c) Au@Cu₂O core–shell, and (d and e) Au@Cu₇S₄ yolk–shell NPs. (f) Diameter distribution histograms for Au@Cu₂O and Au@Cu₇S₄ NPs and they are plotted based on 150 NPs.

Dispersed X-ray Spectroscopy (EDX) spectroscopy was further performed on Au@Cu₇S₄ NPs to reveal the formation of yolk–shell structure. Fig. S4† shows an integrated EDX spectrum of Au@Cu₇S₄ NPs, in which the signals belonging to Au, Cu, and S are detected, indicating the transfer from Au@Cu₂O to Au@Cu₇S₄. The EDX line-scanning across an individual Au@Cu₇S₄ NP is displayed in Fig. 2a, which clearly suggests that Au is strictly confined in the central core area, while S signal is distributed over the whole cross-section of the particle with a slight higher intensity at the shell. It is noteworthy that the Cu line scans contain the excited Cu X-rays from both the sample and the Cu grid. The significant increase in the signal of Cu at the Au core region is largely due to the contribution from the Cu grid, which is greatly enhanced when strong electron scattering from the Au core occurred during the line-scan process.²⁷

Fig. 2 (a) EDX line-scanning profiles across an individual Au@Cu₇S₄ NP. Inset is the corresponding TEM image. (b) XRD spectra of Au@Cu₂O and Au@Cu₇S₄. (c and d) XPS spectra of Au@Cu₇S₄ NPs: (c) survey and (d) Cu 2p.

To provide more evidences to support the formation of Au@Cu₇S₄ from Au@Cu₂O, the X-ray-diffraction (XRD) analysis was also conducted. Before the measurements, the sample suspensions were dip-coated on a silicon substrate and then air-dried to obtain the films, which were used for the characterization. As shown in Fig. 2b, the XRD diffraction peaks from cubic-phase Au are detected in both Au@Cu₂O and Au@Cu₇S₄ samples, indicating the existence of Au. Besides these, the peaks from Cu₂O (PDF file no. 05-0667) and Cu₇S₄ (PDF file no. 23-0598) are also recognized, respectively, for the Au@Cu₂O and Au@Cu₇S₄, though a poor quality in peak signals is presented.

X-ray photoelectron spectroscopy (XPS) technique was further employed to confirm the chemical states of Cu in Au@Cu₇S₄ yolk–shell structures. Fig. 2c shows the survey spectrum of Au@Cu₇S₄ NPs, from which the peaks of Cu, and S elements with atomic concentrations of 9.6 and 5.5%, respectively, are observed. And the Cu/S molar ratio of the sample was calculated to be 1.745, close to the theoretic value of idealized Cu₇S₄. Additionally, a trace amount of O and N elements are also found, which can be attributed to the adsorbed PVP molecules. Since XPS is a surface analysis technique with investigation depth of 2–5 nm and the Au cores are coated by Cu₇S₄, the signal of Au element is not detected.³⁰ Additionally, in Fig. 2d, the highly resolved XPS spectrum of Cu 2p is fitted to four peaks. The main peaks located at 937.1 eV and 951.5 eV can be attributed to the Cu 2p_3/2 and Cu 2p_1/2 of Cu⁺, respectively.^31,32 The shake-up satellite peaks at 933.5 eV and 953.3 eV are evidence for the presence of Cu²⁺.^31,32 By integrating the peak area, the molar ratio of Cu⁺/Cu²⁺ is estimated to be 6.0, well agreement with the value in Cu₇S₄. Based on these results, it can be concluded that the Cu₇S₄ was formed by treating Cu₂O in Na₂S solutions.

The optical properties of the aqueous dispersion containing Au@Cu₇S₄ NPs were examined by UV-vis-NIR spectroscopy, and for a comparison, the Au and Au@Cu₂O NP suspensions were also investigated. In Fig. 3a, the Au NPs shows a typical plasmonic absorption in visible-light range with the peak centered at 520 nm. After coated by the Cu₂O shells, the Au absorption is clearly red-shifted to 566 nm, which can be ascribed to the high refractive index of Cu₇S₄ surrounding the Au cores.³³ Interestingly, this absorption is shifted to around 520 nm again in Au@Cu₇S₄ NP suspensions. This result provides an additional evidence supporting the formation of voids between the Au cores and Cu₇S₄ shells. More importantly, Au@Cu₇S₄ NPs show an enhanced absorption with the increase of wavelength in the NIR region (700–1400 nm). This NIR absorbance of the Au@Cu₇S₄ NPs is mainly attributed to the localized surface plasmon resonances (SPR) of Cu₇S₄ resulted from Cu deficiencies in the structure.⁵

Fig. 3 (a) UV-vis-NIR spectra of Au, Au@Cu₂O, and Au@Cu₇S₄ aqueous dispersions. (b) The photothermal profiles of the aqueous dispersion of Au@Cu₇S₄ NPs with different concentrations in 7 min under the 980 nm laser irradiation. Inset in (b) is the plot of temperature changes at the end of the laser irradiation period vs. nanoparticle concentrations.

The strong SPR absorption of Au@Cu₇S₄ yolk–shell NPs in NIR region motivates us to further investigate their photothermal properties using a 980 nm laser at the plasmon band, which can gain an insight into their potential applications for photothermal cancer therapy. In Fig. 3b, we show the temperature elevation of pure water (as a reference) and Au@Cu₇S₄ aqueous dispersions with different concentrations under the irradiation of 980 nm laser. At laser powder density of 0.51 W cm⁻², with the NP concentration varied from 0.06 to 2.00 g L⁻¹, the temperature elevation of the aqueous dispersions increases from 6.0 to 11.1 °C after 7 min laser irradiation, while the pure water exhibits little heating at the same conditions. These results indicate that the Au@Cu₇S₄ NPs can rapidly and efficiently convert the NIR energy into thermal energy. The final temperature changes at the end of the laser irradiation period as a function of nanoparticle concentrations are plotted in the inset of Fig. 3b. It is clear that the temperature change elevation profile rises up dramatically with the concentration of Au@Cu₇S₄ NPs increasing to 0.25 g L⁻¹ and then flattens out upon the concentration further increasing to 2.0 g L⁻¹. This phenomenon might be attributed to a relatively fast heat loss at high temperature and the logarithmic absorbance dependence of Au@Cu₇S₄ NPs on the fraction of incident radiation.^5,18

A modified method reported by Roper et al. was performed to calculate the photothermal conversion efficiency of the Au@Cu₇S₄ yolk–shell NPs.³⁴ As shown in Fig. 4a, the Au@Cu₇S₄ aqueous dispersions with a concentration of 2.0 g L⁻¹ was kept under a continuous illumination by the 980 nm laser until reaching a steady state temperature. Subsequently, the laser was removed and the dispersions was allowed to cool down naturally. By fitting the measured cooling curve (Fig. 4b), the rate of heat dissipation from the system to the environment could be obtained. According to Roper's report, the photothermal conversion efficiency, η, was calculated using the following equation (eqn (2), detailed in the ESI†):³⁴

where I is incident laser powder (0.3 W), A₉₈₀ is the absorbance of Au@Cu₇S₄ NPs at 980 nm, h is heat transfer coefficient, S is the exposed surface area of the container, T_max is the equilibrium temperature (11.1 °C) of Au@Cu₇S₄ NP aqueous dispersions and T_max,H2O indicates the equilibrium temperature of pure water. Thus, the photothermal conversion efficiency can be calculated to be as high as 63%, which is much higher than the reported values for Cu_xS and Au–Cu_xS hybrid NPs.^5,8,18,20 This outstanding photothermal activity of Au–Cu₇S₄ NPs possibly attributes to multiple reflections of NIR light within the sphere interior voids,²² as illustrated by the inset in Fig. 4a, and the Au NP-driven enhancement in localized surface plasmon resonance of Cu₇S₄.¹⁸ In addition, the comparisons of morphology and absorption spectra shown in Fig. S5† reveal that no obvious difference appeared for Au@Cu₇S₄ yolk–shell NPs after the laser irradiation, indicating their good photostability.

Fig. 4 (a) The photothermal profile over 16 min of an Au@Cu₇S₄ NP aqueous solution (2.00 g L⁻¹) irradiated by laser for 7 min, followed by natural cooling with laser light removed. Inset shows a schematic illustration of multi-reflections within an Au@Cu₇S₄ NPs. (b) Time constant for heat transfer of the system is determined to be τ_s = 112 s.

Conclusions

In summary, we have successfully demonstrated the yolk–shell structured Au@Cu₇S₄ NPs through a wet chemistry approach, which involves the growth of polycrystalline Cu₂O shells outside Au NP cores and subsequent inward replacement of the Cu₂O in a S²⁻ contained solution leading to the formation of hollow Cu₇S₄ shells based on the Kirkendall Effect. Due to localized surface plasmon resonance (SPR) effect and the unique yolk–shell structure, the as-prepared Au@Cu₇S₄ yolk–shell NPs exhibit an enhanced absorption with the increase of wavelength in near-infrared (NIR) region. Under the irradiation of a 980 nm laser, the materials show a high photothermal conversion efficiency of 63%, which results in the increase of the temperature of their aqueous dispersions by 11.1 °C in 7 min. This feature makes Au@Cu₇S₄ NPs potential candidates for photothermal therapy applications. Furthermore, the designed Au NP-contained yolk–shell structure may also hold great promise for drug delivery.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (No: 51372169).

Notes and references

1. W. C. Chueh, C. Falter, M. Abbott, D. Scipio, P. Furler, S. M. Haile and A. Steinfeld, Science, 2010, 330, 1797–1801.
2. L. Liu, S. Ouyang and J. Ye, Angew. Chem., Int. Ed., 2013, 52, 6689–6693.
3. X. Meng, T. Wang, L. Liu, S. Ouyang, P. Li, H. Hu, T. Kako, H. Iwai, A. Tanaka and J. Ye, Angew. Chem., Int. Ed., 2014, 53, 11478–11482.
4. F. Wang, C. Li, H. Chen, R. Jiang, L.-D. Sun, Q. Li, J. Wang, J. C. Yu and C.-H. Yan, J. Am. Chem. Soc., 2013, 135, 5588–5601.
5. Q. Tian, F. Jiang, R. Zou, Q. Liu, Z. Chen, M. Zhu, S. Yang, J. Wang, J. Wang and J. Hu, ACS Nano, 2011, 5, 9761–9771.
6. K. Dong, Z. Liu, Z. Li, J. Ren and X. Qu, Adv. Mater., 2013, 25, 4452–4458.
7. Q. Tian, J. Hu, Y. Zhu, R. Zou, Z. Chen, S. Yang, R. Li, Q. Su, Y. Han and X. Liu, J. Am. Chem. Soc., 2013, 135, 8571–8577.
8. B. Li, Q. Wang, R. Zou, X. Liu, K. Xu, W. Li and J. Hu, Nanoscale, 2014, 6, 3274–3282.
9. S. Linic, P. Christopher and D. B. Ingram, Nat. Mater., 2011, 10, 911–921.
10. J. Qiu and W. D. Wei, J. Phys. Chem. C, 2014, 118, 20735–20749.
11. H. Chen, L. Shao, T. Ming, Z. Sun, C. Zhao, B. Yang and J. Wang, Small, 2010, 6, 2272–2280.
12. L. Au, D. Zheng, F. Zhou, Z.-Y. Li, X. Li and Y. Xia, ACS Nano, 2008, 2, 1645–1652.
13. J. Chen, D. Wang, J. Xi, L. Au, A. Siekkinen, A. Warsen, Z.-Y. Li, H. Zhang, Y. Xia and X. Li, Nano Lett., 2007, 7, 1318–1322.
14. H. Yuan, A. M. Fales and T. Vo-Dinh, J. Am. Chem. Soc., 2012, 134, 11358–11361.
15. X. Huang, S. Tang, X. Mu, Y. Dai, G. Chen, Z. Zhou, F. Ruan, Z. Yang and N. Zheng, Nat. Nanotechnol., 2011, 6, 28–32.
16. N. W. S. Kam, M. O'Connell, J. A. Wisdom and H. Dai, Proc. Natl. Acad. Sci. U. S. A., 2005, 102, 11600–11605.
17. L. Feng, X. Yang, X. Shi, X. Tan, R. Peng, J. Wang and Z. Liu, Small, 2013, 9, 1989–1997.
18. X. Ding, C. H. Liow, M. Zhang, R. Huang, C. Li, H. Shen, M. Liu, Y. Zou, N. Gao, Z. Zhang, Y. Li, Q. Wang, S. Li and J. Jiang, J. Am. Chem. Soc., 2014, 136, 15684–15693.
19. Q. Xiao, X. Zheng, W. Bu, W. Ge, S. Zhang, F. Chen, H. Xing, Q. Ren, W. Fan, K. Zhao, Y. Hua and J. Shi, J. Am. Chem. Soc., 2013, 135, 13041–13048.
20. Q. Tian, M. Tang, Y. Sun, R. Zou, Z. Chen, M. Zhu, S. Yang, J. Wang, J. Wang and J. Hu, Adv. Mater., 2011, 23, 3542–3547.
21. Y. Zhao, H. Pan, Y. Lou, X. Qiu, J. Zhu and C. Burda, J. Am. Chem. Soc., 2009, 131, 4253–4261.
22. H. Li, Z. Bian, J. Zhu, D. Zhang, G. Li, Y. Huo, H. Li and Y. Lu, J. Am. Chem. Soc., 2007, 129, 8406–8407.
23. J. Liu, S. Z. Qiao, J. S. Chen, X. W. Lou, X. Xing and G. Q. Lu, Chem. Commun., 2011, 47, 12578–12591.
24. H. Cao, X. Qian, C. Wang, X. Ma, J. Yin and Z. Zhu, J. Am. Chem. Soc., 2005, 127, 16024–16025.
25. S. Jiao, L. Xu, K. Jiang and D. Xu, Adv. Mater., 2006, 18, 1174–1177.
26. C.-H. Kuo, Y.-T. Chu, Y.-F. Song and M. H. Huang, Adv. Funct. Mater., 2011, 21, 792–797.
27. L. Zhang, D. A. Blom and H. Wang, Chem. Mater., 2011, 23, 4587–4598.
28. X. Ji, X. Song, J. Li, Y. Bai, W. Yang and X. Peng, J. Am. Chem. Soc., 2007, 129, 13939–13948.
29. C.-H. Kuo, T.-E. Hua and M. H. Huang, J. Am. Chem. Soc., 2009, 131, 17871–17878.
30. G. Liu, F. He, J. Zhang, L. Li, F. Li, L. Chen and Y. Huang, Appl. Catal., B, 2014, 150–151, 515–522.
31. R. Cai, J. Chen, J. Zhu, C. Xu, W. Zhang, C. Zhang, W. Shi, H. Tan, D. Yang, H. H. Hng, T. M. Lim and Q. Yan, J. Phys. Chem. C, 2012, 116, 12468–12474.
32. X. Jiang, Y. Xie, J. Lu, W. He, L. Zhu and Y. Qian, J. Mater. Chem., 2000, 10, 2193–2196.
33. P. K. Jain, I. H. El-Sayed and M. A. El-Sayed, Nano Today, 2007, 2, 18–29.
34. D. K. Roper, W. Ahn and M. Hoepfner, J. Phys. Chem. C, 2007, 111, 3636–3641.

---

Footnotes

† Electronic supplementary information (ESI) available: Experimental procedures and additional experimental results. See DOI: 10.1039/c5ra19055j

‡ Both J. Zhang and G. Liu have contributed equally towards the work.

---