Selective etching of binary nanoparticle superlattices via thermally induced asymmetric ligand evolution

Abstract

We report a method to selectively etch either component in NaYF₄/Fe₃O₄ binary nanoparticle superlattices using a single etchant. Thermally-induced evolution of oleate ligands on the NaYF₄ nanoparticles reverses their relative etching susceptibility in oxalic acid, yielding non-close-packed nanoparticle arrays with tunable architectures.

---

The self-assembly of colloidal nanoparticles (NPs) provides a powerful bottom-up route to organize nanoscale building blocks into ordered superlattices with collective and emergent properties. Such superlattices have attracted intense interest for applications in electronic, optical, and mechanical properties, where long-range periodicity and structural coherence are essential.^1–3 Importantly, the properties of these materials can be precisely tuned by the choice and number of NP building blocks, the length and functionality of the surface ligands, and the symmetry that governs their coupling within the lattice.⁴

Beyond conventional close-packed architectures, non-close-packed (NCP) ordered arrays—in which NPs remain periodically arranged but are separated by larger, well-defined distances—offer distinct advantages, including enhanced accessibility, porosity and electromagnetic field concentration. These features make NCP arrays particularly attractive as surface-enhanced Raman scattering (SERS) substrates⁵ and as templates for quantum dot or plasmonic device fabrication.⁶ Despite several elegant demonstrations based on highly specific interactions or finely balanced assembly conditions,⁷ a general and programmable strategy to access ordered NCP NP arrays is still lacking.

Selective removal of one component from binary NP superlattices (BNSLs) by chemical etching represents a particularly powerful route to transform dense arrays into ordered NCP architectures. In practice, however, most reported strategies rely on the intrinsic chemical differences between the two components and require the use of different etchants to achieve selectivity.^5,8 A representative example is reported in Fe₃O₄/Au BNSLs, where Fe₃O₄ NPs are etched by acids while Au NPs require a cyanide solution such as KCN.⁵ Although effective, this etchant-dependent paradigm is inherently complex, limits programmability, and offers little insight into the molecular origin of etching selectivity. A simpler and mechanistically transparent strategy that enables selective etching using a single etchant—while retaining full structural order—therefore remains highly desirable for post-assembly structural editing.

Here we introduce an alternative, ligand reorganization–governed strategy for selective etching of BNSLs using a single etchant. Using a NaYF₄/Fe₃O₄ system, we exploit the thermally induced chemical transformation and crosslinking of oleate ligands on NaYF₄ NPs, enabled by their low thermal conductivity and nanoscale heat accumulation under thermal treatment.⁹ As a result, the ligand shell on NaYF₄ evolves from a loosely bound molecular layer into a rigid, interconnected network. When the degree of ligand crosslinking is low, NaYF₄ remains susceptible to etching in oxalic acid; when crosslinking is sufficiently developed, acid penetration is effectively suppressed. In contrast, the ligand shell on the Fe₃O₄ NPs remains largely unchanged under the same conditions. This difference allows the relative etching susceptibility of the two components to be reversed simply by tuning the thermal treatment time, without changing the etching chemistry. Spectroscopic analyses reveal a molecular-level reconstruction of the ligand shell as the origin of this behavior, establishing a new conceptual basis for programmable post-assembly structural editing of nanocrystal superlattices.

Monodisperse NaYF₄ and Fe₃O₄ nanospheres (NSs) were synthesized following established protocols.¹⁰ For constructing AB-type BNSLs, Fe₃O₄ NSs (diameter: 13.1 ± 0.3 nm) and NaYF₄ NSs (diameter: 33.7 ± 0.1 nm) were co-assembled at the liquid–air interface via slow evaporation of hexane (Fig. S1a and c).¹¹ This process yields two-dimensional membranes with long-range order, which can be readily transferred onto carbon-coated transmission electron microscopy (TEM) grids for structural characterization and post-assembly processing (Fig. 1a). Low-magnification TEM images reveal long-range periodic ordering of the AB-type superlattice (Fig. 1b and Fig. S3), while the corresponding small-angle electron diffraction (SAED) pattern confirms crystalline coherence across the entire lattice. High-angle annular dark-field scanning TEM (HAADF-STEM) images combined with elemental mapping unambiguously assign Fe and Y to two interpenetrating sublattices, verifying the formation of a binary array (Fig. 1c).

Fig. 1 (a) Schematic illustration of the method. (b) TEM image, SAED pattern (inset), and (c) HAADF-STEM image and elemental mappings of AB-type BNSLs assembled from NaYF₄ and Fe₃O₄ NSs. (d) TEM image and (e) HAADF-STEM image and corresponding elemental mappings of an NCP Fe₃O₄ array. (f) TEM image, (g) HAADF-STEM image, and corresponding elemental mappings of an NCP NaYF₄ array.

By adjusting the thermal treatment time prior to etching, the target of oxalic acid etching can be reversibly switched. After a short duration of thermal treatment (3 h), NaYF₄ is preferentially removed due to its intrinsically lower acid resistance compared with Fe₃O₄ (Fig. S4), yielding an NCP Fe₃O₄ array. Large-area TEM images show a periodic square array (Fig. 1d), while HAADF-STEM and Fe/O elemental mapping confirm the exclusive retention of Fe₃O₄ in the remaining lattice (Fig. 1e). The average diagonal center-to-center spacing is 36.6 nm (Fig. S5), consistent with the diameter of the removed NaYF₄ NSs plus the oleate ligand shell. Both the particle size and lattice registry of Fe₃O₄ remain unchanged, indicating that oxalic acid selectively removes NaYF₄ without disrupting the Fe₃O₄ sublattice.

Conversely, extending the thermal treatment to 6 h reverses the etching selectivity. Under identical oxalic acid conditions, Fe₃O₄ NSs are now preferentially removed, while NaYF₄ NSs remain intact, producing a complementary NCP NaYF₄ array. Low-magnification TEM images confirm the formation of an ordered NaYF₄ array (Fig. 1f and Fig. S6), and HAADF-STEM with Y/F elemental mapping verifies the exclusive presence of NaYF₄ NSs in the remaining lattice (Fig. 1g). Notably, the etching process is not instantaneous but evolves progressively with time: statistical TEM analysis after 3 h and 6 h of thermal treatment reveals a continuous, stepwise removal of the susceptible component until complete elimination is achieved (Fig. S7). This inversion of etching susceptibility establishes an “alternative etching” regime governed solely by the thermal treatment time of the superlattice, rather than by changing the chemical identity of the etchant.

To rationalize the “alternative etching” behavior observed in Fig. 1, we quantified the time required for complete removal of NaYF₄ and Fe₃O₄ NSs as a function of prior thermal treatment (Fig. 2a). The etching kinetics of Fe₃O₄ are largely insensitive to the thermal treatment time, whereas the etching kinetics of NaYF₄ increase systematically with increasing thermal treatment duration. The plot reveals a crossover at 4 h, where the relative etching susceptibility of the two components is reversed. Without thermal treatment, NaYF₄ is etched much faster than Fe₃O₄ and the superlattice collapses during etching (Fig. S4). A moderate treatment of 3 h stabilizes the lattice while maintaining preferential removal of NaYF₄, yielding intact Fe₃O₄ NCP arrays. Extending the thermal treatment to 6 h drives the system beyond the crossover, where Fe₃O₄ is preferentially etched and the NaYF₄ sublattice remains intact. This inversion originates from the intrinsic difference in thermal conductivity between the two cores, which leads to programmable modification of the surface states. We therefore probe the molecular origin of this behavior by examining how thermal treatment reconstructs the ligand shells of both components.

Fig. 2 (a) Plot of the time required for complete dissolution of NaYF₄ and Fe₃O₄ NSs as a function of thermal treatment duration. (b) Expanded ¹H NMR spectra of the NaYF₄ NSs. (c) FTIR spectra and (d) expanded FTIR spectra of the NaYF₄ NSs. (e) and (f) C 1s XPS spectra of the NaYF₄ and Fe₃O₄ NSs, respectively.

To elucidate the molecular origin of the thermally induced etching reversal, we first investigated the evolution of surface ligands on NaYF₄ NSs by ¹H nuclear magnetic resonance (NMR) spectroscopy. The ligand structures were analyzed using single-component NaYF₄ samples subjected to different thermal treatments (0 h and 6 h), with spectra acquired in CDCl₃ after redispersing the thermally treated NSs from silicon substrates (Fig. 2b and Fig. S8). The ¹H NMR spectrum of the as-prepared NaYF₄ NSs shows characteristic signals of oleate ligands. After 6 h thermal treatment, the alkene resonance at δ = 5.34 ppm, assigned to –HC=CH– protons, disappears completely, indicating cleavage of the C=C bond in surface-bound oleate.^12,13 This transformation enables ligand crosslinking and shell densification, forming a compact surface layer that suppresses acid penetration and underlies the enhanced etching resistance.

Fourier transform infrared (FTIR) spectroscopy reveals a pronounced reorganization of the ligand shell on NaYF₄ NSs upon thermal treatment. As shown in Fig. 2c and d, pristine NaYF₄ NSs exhibit characteristic oleate features, including CH₂ stretching bands at 2924 and 2854 cm⁻¹ and a weak =C–H vibration at 3007 cm⁻¹, consistent with surface-bound oleic acid ligands. After thermal treatment, the most pronounced changes occur in the carboxylate region. In the pristine sample, the bands at 1550 and 1485 cm⁻¹ are associated with the asymmetric (ν_as) and symmetric (ν_s) stretching vibrations of the carboxylic group of the bound oleic acid. After thermal treatment, these bands shift to 1562 and 1457 cm⁻¹. Accordingly, the separation Δν = ν_as − ν_s increases from 65 cm⁻¹ to 105 cm⁻¹.^14–16 Fig. S9 shows the FTIR spectra of NaYF₄ NSs following thermal treatment for different durations (0, 3, 6 and 8 h). With increasing annealing time, the ν_as band gradually shifts to higher wavenumbers, whereas the ν_s band moves to lower wavenumbers, leading to a progressive increase in Δν. Such an increase in Δν is commonly associated with a change in the carboxylate coordination environment, consistent with a transition from relatively flexible surface binding to a more compact and constrained coordination state. This evolution suggests that thermal treatment induces chemical reorganization and intermolecular coupling of oleate ligands on the NaYF₄ surface.

X-ray photoelectron spectroscopy (XPS) provides direct evidence for thermally induced ligand reconstruction on NaYF₄ NSs and highlights its contrast with Fe₃O₄ NSs. In the C 1s spectra of NaYF₄ (Fig. 2e), thermal treatment induces a systematic decrease of the C–O component accompanied by an increase of the C=O contribution, which supports the formation of a more chemically constrained surface layer that correlates with the enhanced acid resistance of NaYF₄ after thermal treatment. In contrast, the C 1s spectra of Fe₃O₄ NSs (Fig. 2f) show no signatures of ligand crosslinking under the same thermal treatment. Simultaneously, the Fe 2p_3/2 and Fe 2p_1/2 peaks (Fig. S10c) shift slightly toward higher binding energies after thermal treatment, indicating partial oxidation of Fe²⁺ to Fe³⁺.^3,6 Thus, Fe₃O₄ primarily undergoes core-level oxidation rather than surface–ligand reconstruction. This stark difference demonstrates that only NaYF₄ experiences thermally driven ligand reorganization and densification, providing the molecular basis for the programmable and reversible etching selectivity observed in the binary superlattices.

The spectroscopic evidence and etching trends in Fig. 2 converge on a unified mechanism: nanoscale heat accumulation in NaYF₄ NSs triggers ligand intermolecular crosslinking and densification of the oleate shell, transforming a permeable organic corona into a compact barrier against acid attack.⁹ The surface ligands on the Fe₃O₄ NSs, in contrast, do not undergo comparable reconstruction under the same thermal conditions and remain relatively accessible to the etchant.^3,6 The thermal conductivity of NaYF₄ (0.5 W m⁻¹ K⁻¹), markedly lower than that of Fe₃O₄ (6 W m⁻¹ K⁻¹), results in preferential heat localization during thermal treatment.^17,18 This divergence defines the molecular origin of the programmable and reversible etching selectivity in the binary superlattices.

Crucially, this asymmetric ligand evolution arises from the intrinsic difference in thermal conductivity and heat dissipation between the two inorganic cores, which dictates how efficiently thermal energy is delivered to the surface ligands. As schematically illustrated in Fig. 3, the superlattice therefore encodes a thermally programmable surface chemistry: at short thermal treatment times NaYF₄ is preferentially etched, whereas prolonged thermal treatment renders NaYF₄ acid-resistant and shifts the etching to Fe₃O₄. In this way, thermal treatment time is translated into controllable interfacial reactivity, enabling reversible switching of etching selectivity by engineering ligand-shell evolution rather than altering the etchant itself.

Fig. 3 Schematic illustrating the inversion of etching selectivity in binary superlattices.

To evaluate the generality of the ligand-reorganization–governed etching mechanism, we extended this strategy to a second binary system composed of nanodumbbells (NDS) and NSs (Fig. 4a).¹⁰ Oleate-capped β-NaYF₄:Yb/Er@NaGdF₄@NaNdF₄ NDs, defined by the diagonal head size (D) and waist diameter (d) (Fig. S2), were co-assembled with Fe₃O₄ NSs (21.1 ± 0.1 nm; Fig. S1b) using the same liquid–air interfacial method. The two components organize into a hexagonal AB₂-type superlattice, in which each ND is surrounded by six NSs, while each nanosphere is coordinated by three NDs (Fig. 4a). The long-range order is confirmed by large grain sizes and the corresponding SAED pattern (Fig. 4b and Fig. S11). HAADF-STEM imaging and elemental mapping resolve that Fe₃O₄ NSs occupy the interstitial voids between adjacent NDs (Fig. 4b).

Fig. 4 (a) Schematic of the assembly and selective etching of AB₂-type superlattices formed from β-NaYF₄:Yb/Er@NaGdF₄@NaNdF₄ NDs and Fe₃O₄ NSs. (b) HAADF-STEM image (inset: SAED pattern) and corresponding elemental mappings of the AB₂-type superlattices. (c) HAADF-STEM image of the honeycomb-packed NS array, with the corresponding SAED pattern (top right) and elemental mapping (bottom right). (d) HAADF-STEM image of the hexagonally packed ND array, with the corresponding SAED pattern (top right) and elemental mapping (bottom right).

After thermal treatment, the same oxalic-acid etching protocol selectively removes one component from the dumbbell–sphere superlattice, yielding ordered NCP arrays. Preferential etching of the NDs produces a honeycomb-packed Fe₃O₄ lattice (Fig. 4c and Fig. S12),⁷ whose long-range order is preserved as evidenced by SAED, while Fe mapping confirms that only Fe₃O₄ NSs remain. The Fe₃O₄ NSs size is essentially unchanged, and the interparticle spacing matches the waist dimension of the removed NDs (Fig. S12).

Conversely, reversing the etching selectivity removes the Fe₃O₄ NSs while preserving the NDs, yielding an ordered hexagonal NCP ND array, as confirmed by SAED and Gd elemental mapping (Fig. 4d). After etching, the diagonal head size decreases from 43.0 to 37.3 nm, and the initially sharp hexagonal heads become visibly more rounded, suggesting facet-selective dissolution that progressively smooths the dumbbell geometry during etching, while the overall hexagonal registry is maintained (Fig. S13). Similarly, the etching process evolves progressively with time, as evidenced by the temporal statistics in Fig. S14. This establishes a general principle: etching selectivity is governed by particle composition rather than shape. By exploiting nanoscale heat accumulation in NaYF₄ NPs, thermal energy is translated into ligand reconstruction and programmable interfacial chemistry, enabling reversible and morphology-independent control of etching sequences in binary superlattices.

We demonstrate a simple and controllable approach to selectively etch BNSLs. Thermally-induced reorganization and densification of the ligand shell on NaYF₄ NPs inverts the relative etching preference of the two components without changing the etchant. The concept’s generality across different NaYF₄ morphologies highlights its potential as a versatile platform for nanoscale design.

This work was supported by the NSFC (52333013, 52522317, 52503361, 22025501, and 22088101) and the Shanghai Municipal Science and Technology Project (24TS1410200, 23JC1400100, 2024ZDSYS02, 24JD1400700, and 24QA2700800).

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information is available. See DOI: https://doi.org/10.1039/d6cc00778c.

References

1. Y. Kang, X. Ye, J. Chen, Y. Cai, R. E. Diaz, R. R. Adzic, E. A. Stach and C. B. Murray, J. Am. Chem. Soc., 2013, 135, 42–45.
2. F. Schulz, O. Pavelka, F. Lehmkühler, F. Westermeier, Y. Okamura, N. S. Mueller, S. Reich and H. Lange, Nat. Commun., 2020, 11, 3821.
3. A. Dreyer, A. Feld, A. Kornowski, E. D. Yilmaz, H. Noei, A. Meyer, T. Krekeler, C. Jiao, A. Stierle, V. Abetz, H. Weller and G. A. Schneider, Nat. Mater., 2016, 15, 522–528.
4. Y. Liu, M. Klement, Y. Wang, Y. Zhong, B. Zhu, J. Chen, M. Engel and X. Ye, J. Am. Chem. Soc., 2021, 143, 16163–16172.
5. T. Udayabhaskararao, T. Altantzis, L. Houben, M. Coronado-Puchau, J. Langer, R. Popovitz-Biro, L. M. Liz-Marzán, L. Vuković, P. Král, S. Bals and R. Klajn, Science, 2017, 358, 514–518.
6. K. Santhosh, O. Bitton, L. Chuntonov and G. Haran, Nat. Commun., 2016, 7, 11823.
7. M. P. Boneschanscher, W. H. Evers, J. J. Geuchies, T. Altantzis, B. Goris, F. T. Rabouw, S. A. P. van Rossum, H. S. J. van der Zant, L. D. A. Siebbeles, G. Van Tendeloo, I. Swart, J. Hilhorst, A. V. Petukhov, S. Bals and D. Vanmaekelbergh, Science, 2014, 344, 1377–1380.
8. M. Piotrowski, Z. Ge, X. Han, Y. Wang, A. K. Bandela and U. Thumu, Nanoscale, 2023, 15, 5188–5192.
9. Y. Cai, M. Lu, X. Qin, D. Jin and J. Zhou, Nat. Commun., 2025, 16, 4927.
10. Z. Zheng, H. Wang, Y. Gao, S. Wan, L. Lv, J. Yang, D. Yang, Y. Ling, T. Li and A. Dong, ACS Nano, 2025, 19, 29788–29797.
11. A. Dong, J. Chen, P. M. Vora, J. M. Kikkawa and C. B. Murray, Nature, 2010, 466, 474–477.
12. H. Hu, M. Yu, F. Li, Z. Chen, X. Gao, L. Xiong and C. Huang, Chem. Mater., 2008, 20, 7003–7009.
13. Z. Chen, H. Chen, H. Hu, M. Yu, F. Li, Q. Zhang, Z. Zhou, T. Yi and C. Huang, J. Am. Chem. Soc., 2008, 130(10), 3023–3029.
14. K. I. Hadjiivanov, D. A. Panayotov, M. Y. Mihaylov, E. Z. Ivanova, K. K. Chakarova, S. M. Andonova and N. L. Drenchev, Chem. Rev., 2021, 121, 1286–1424.
15. L. Wang and Y. Li, Nano Lett., 2006, 6, 1645–1649.
16. Z. Wang, A. S. S. Singaravelu, R. Dai, Q. Nian, N. Chawla and R. Y. Wang, Angew. Chem., Int. Ed., 2020, 59, 9556–9563.
17. N. W. Park, W. Y. Lee, J. A. Kim, K. Song, H. Lim, W. D. Kim, S. G. Yoon and S. K. Lee, Nanoscale Res. Lett., 2014, 9, 96.
18. J. M. Gonçalves, A. R. N. Bastos, S. J. L. Ribeiro, L. D. Carlos, R. L. L. Longo, J. M. A. Caiut and R. A. S. Ferreira, Nanoscale Adv., 2024, 6(5), 1486–1496.

---