Poly(Ethylene Oxide) Mediated Synthesis of Sub-100-nm Aluminum Nanocrystals for Deep Ultraviolet Plasmonic Nanomaterials

Open AccessCCS ChemistryCOMMUNICATION1 Aug 2020Poly(Ethylene Oxide) Mediated Synthesis of Sub-100-nm Aluminum Nanocrystals for Deep Ultraviolet Plasmonic Nanomaterials Shuang Yang, Shaoyong Lu, Yang Li, Hua Yu, Linxia He, Tianmeng Sun, Bai Yang and Kun Liu Shuang Yang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Shaoyong Lu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Yang Li State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Hua Yu State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Linxia He State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 , Tianmeng Sun The First Bethune Hospital and Institute of Immunology, Jilin University, Changchun 130021 , Bai Yang State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130012 and Kun Liu *Corresponding author: E-mail Address: [email protected] State Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin University, Changchun 130012 State Key Laboratory of Applied Optics, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, Changchun 130012 https://doi.org/10.31635/ccschem.020.202000141 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Aluminum nanocrystals (Al NCs) are sustainable plasmonic nanomaterials with unique localized surface plasmonic resonance (LSPR) in the ultraviolet (UV) region. Chemical synthesis of sub-100-nm Al NCs remains a considerable challenge due to the lack of effective ligands to control their growth. Here, we describe a precise size-controlled synthesis of small colloidal Al NCs (25–100 nm) with strong and tunable LSPR peak from 250 to 372 nm in the UV spectral region by the use of poly(ethylene oxide) (PEO) as a polydentate surface ligand. The LSPR band matched well with the numerical simulation results. Additionally, the molecular weight of the PEO played an essential role in tuning the size of Al NCs. The PEO showed a strong affinity with Al {111} crystal facets, resulting in octahedral- and prism-shaped Al NCs. Owing to the passive oxide layer generated on the surface of Al NCs, their LSPR peak positions showed negligible changes after storage in tetrahydrofuran for 4 months. The passivation layer also impeded the metal Al core to react with deionized water. Further, the use of biocompatible PEO ligand and the subsequent generation of the sub-100-nm size of the Al NCs pave a path for bioapplications of such NCs. Download figure Download PowerPoint Introduction Ultraviolet (UV) plasmonics has drawn growing attention in recent years for its unique application in UV surface-enhanced Raman spectroscopy (UV SERS), photocatalysis, and surface-enhanced fluorescence (SEF).1–11 However, the most common plasmonic materials, for example, gold, silver, and copper, only cover the spectral range from the visible region to near-infrared (NIR) region, because of the presence of interband transitions in the UV region for which the excitation of localized surface plasmon resonance (LSPR) is inhibited.12,13 Instead, aluminum nanomaterials possess a much broader spectral range of LSPR from UV to NIR region and hold great promise as UV plasmonic nanomaterials to fill in for the shortage of noble metal.14–17 The exploration of UV plasmonic nanomaterial is highly rewarding in that the UV region, especially at wavelength < 300 nm, is mostly correlated with the electronic transitions of frontier orbitals of small molecules, chromophoric segments of large macromolecules and biological molecules, including protein residues, DNA bases, and others.5,12,18–21 The detailed information and signals of these molecules are difficult to identify by visible, and NIR SERS, but could be enhanced dramatically by UV SERS because the Raman scattering cross sections scale is approximately in the order of fourth power magnitude of laser frequency.19,22–25 In addition, Al nanoparticles with UV plasmonic have shown excellent efficiency for plasmon-enhanced photocatalysis.7 Al nanomaterials with UV-LSPR require their sizes to be < 120 nm.26,27 However, it is difficult to fabricate sub-100 nm Al nanostructures by top-down lithographic techniques due to the limited resolution, high cost, and time consumption.28–32 Other laser-based or pulsed sonoelectrochemical methods have yielded sub-100 nm Al nanoparticles but with no distinct LSPR peaks in the UV region due to their broad size distribution and/or sever aggregations caused by the lack of surface ligands.33 To date, the preparation of sub-100 nm Al nanocrystals (NCs) with strong plasmonic resonance in the UV region is still a great challenge.33 Ligand-controlled wet-chemistry synthesis of metal nanoparticles is one mature way to control their size and morphology. During the syntheses, organic ligands could control the nucleation and preferential growth facets of nanoparticles effectively via organic−inorganic interface absorption on specific crystal facets.34–41 Recently, our group reported the use of an end-group functionalized polymer, cumyl dithiobenzoate-terminated polystyrene (CDTB-PS), as a surface ligand for the morphology- and size-controlled synthesis of monodisperse Al NCs with a wide size range from 130 to 250 nm.42 However, Al NCs in this size range exhibited multiple plasmonic peaks, including dipolar plasmonic resonances in the vis–NIR region and high-order plasmonic resonance in the UV region. Accordingly, Al NCs with a size < 100 nm could not be obtained by using CDTB-PS ligands, since there is only one functional end group on each polymer chain to control their growth. Thus, for small NCs, ligands with strong binding affinity to the Al surface are desired.43,44 Herein, we report a wet-chemical synthesis of sub-100-nm Al NCs with strong size-tunable LSPR peaks from 250 to 372 nm (UV region) induced by utilizing poly(ethylene oxide) (PEO) as a polydentate surface ligand. The size range of Al NCs was controlled precisely from 25 to 100 nm by adjusting the concentration of titanium (IV) isopropoxide (Ti(i-PrO)4) as a catalyst. More remarkably, the Al NCs with the size of 25 nm showed a narrow LSPR peak with a full width of half maximum (FWHM) at 80 nm. Meanwhile, the ether bonds (C–O–C) in the backbone of PEO played a key role in controlling the nucleation and growth of NCs, showing a strong affinity with Al {111} facet. The PEO with different number average molecular weights (Mn) showed different size-controlled abilities. Also, the effects of the passivation oxide layer on the Al NCs for storage in tetrahydrofuran (THF) and in deionized (DI) water were studied. Results and Discussion Small colloidal Al NCs were prepared by the decomposition of 1-methylpyrrolidine alane (H3Al(1-MP) precursor catalyzed by Ti(i-PrO)4 in a THF solution containing PEO. The size of Al NCs could be adjusted by varying the concentration of Ti(i-PrO)4 with a constant concentration of H3Al(1-MP) (40 mM) and PEO (11.5 mg/mL, Mn = 10 kg/mol). As the concentration of Ti(i-PrO)4 increased from 8.0 to 560 μM, the as-synthesized solutions changed gradually from a cloudy gray to dark blue, as shown in Figure 1a. The variation of the solution color, which is mainly attributed to the scattering of Al NCs, indicated that the size of Al NCs decreased with the increasing Ti(i-PrO)4, consistent with previous results.42 When the Al NC solutions were diluted 10 times, as shown in (Figure 1b), there was a clear demonstration that the turbidity of the solutions decreased gradually as the size of the Al NCs became smaller. The dynamic light scattering (DLS) study of the solutions showed that when the concentration of Ti(i-PrO)4 increased from 8.0 to 560 μM, the average hydrodynamic diameters of Al NCs varied from 102.9 to 46.7 nm ( Supporting Information Figure S1). The size of the Al NCs was determined by transmission electron microscopy (TEM) analysis (Figure 1d and 1e; Supporting Information Figure S2). Figures 1d and 1e display the representative TEM images of Al NCs prepared with the highest and lowest Ti(i-PrO)4 of 560 and 12 μM, respectively, yielding Al NCs with a mean size, D, (defined as the diameter of their two-dimensional projection in TEM images), of 25.3 ± 4.8 and 98.6 ± 23.4 nm, respectively. The slight increase of their hydrodynamic diameter determined by DLS compared to the TEM analysis is mainly due to the presence of PEO surface ligands. The TEM studies of the Al NCs synthesized with different Ti(i-PrO)4 concentrations confirmed that the size of Al NCs decreased with increasing the Ti(i-PrO)4 concentration, as shown in (Figure 1c and Supporting Information Figure S2). Figure 1 | (a) Final states of the THF solutions of Al NCs synthesized with a series of concentrations of Ti(i-PrO)4 from 8.0-560 μM (arrow direction). (b) The solutions in the green area after 10 times dilution. (c) The average size of Al NCs (D) as a function of the concentrations of Ti(i-PrO)4 (from 98.6 ± 23.4 to 25.3 ± 4.8 nm). (d–e) Low-magnification TEM images of Al NCs with D of (d) 25.3 ± 4.8 and (e) 98.6 ± 23.4 nm. (f) SEM images of Al NCs with the D of 98.6 ± 23.4 nm. Inset: high-magnification SEM image of the same Al NCs. (g–i) HR-TEM images of individual Al NC with D of (g and h) 25.3 ± 4.8 and (i) 98.6 ± 23.4 nm. The image (h) was taken from the corner of small Al NC in (g). (j) Selected area electron diffraction pattern of the Al NC in (i). (k) PXRD pattern of the Al NC samples with average sizes of 25.3 ± 4.8, 56.2 ± 18.1, and 98.6 ± 23.4 nm. The intensity of 25 nm Al NC was magnified by 15 times. PDF Card: Al (PDF# 04-0787), Al2O3 (PDF# 04-002-8135). Download figure Download PowerPoint A close inspection of the TEM images of Al NCs in (Figures 1d and 1e) revealed that the shapes of Al NCs were mainly consisted of {111} facet-terminated octahedrons, prisms, and a small fraction of pentagonal bipyramids, assigned to single, singly-twinned, and fivefold-twinned crystal, respectively. The resultant mixture of Al NCs with different shapes was consistent with our previous report of Al NCs synthesized by using CDTB-PS as a surface ligand.42 In addition, the small Al NCs in Figure 1d possessed more rounded and truncated shapes, while the large Al NCs show shaper edges and corners ( Supporting Information Figure S2), which were demonstrated further by scanning electron microscopy (SEM) images (Figure 1f). Upon exposure to air, a passivation oxide layer was formed on the surface of the Al NCs. This passivation layer is essential for their practical applications because of its ability to prevent further oxidization of the inner metallic core of the Al NCs and facilitate their surface modification, as well as functionalization. High-resolution TEM (HR-TEM) images of both small (Figures 1g and 1h) and large Al NCs (Figure 1i) demonstrated that both types of Al NCs possessed highly crystalline core covered by an amorphous self-limiting oxide shell with a thickness of ∼ 3.5 nm. The measured lattice fringe of the NC (2.32 Å) in Figure 2h was similar to the {111} lattice spacing (2.34 Å) for bulk Al with a face-centered cubic (fcc) lattice structure. The powder X-ray diffraction (PXRD) patterns of Al NCs with different D (Figure 1k) confirmed further the high crystallinity of the Al NCs. The FWHM of Al NC {111} diffraction peaks became narrower for larger Al NCs, owning to the increased size of the crystalline core (See Supporting Information Figure S3). The Al NCs with an average D of 98 ± 23.4 nm showed a high ratio of {111} to {100} peaks of 26.42 (the value of 2.13 for bulk Al), indicating the significantly high enrichment of {111} facets for the large Al NCs. Besides, no peaks of crystalline Al2O3 were observed, which suggested further that the passivation oxide layer was amorphous. Figure 2 | Experimental and FDTD-simulated extinction spectra of different size Al NC aqueous solutions. (a) Experimental extinction spectra of different size Al NC aqueous solutions. (b) The size of Al NCs as a function of the corresponding extinction peaks. (c–h) FDTD simulation extinction spectra of different size Al NCs in water (blue line: simulated extinction spectra of the prism, green line: simulated extinction spectra of the octahedron, red line: ensembled extinction spectra of both prism and octahedron, black dash line: extinction spectra of corresponding size Al NC). The D of models in (c) 25, (d) 30 , (e) 45, (f) 60, (g) 80, and (h) 100 nm corresponding to shape and measured size in TEM images. Download figure Download PowerPoint The LSPR peak of Al NCs is determined by their size, shape, and the local refractive index of surrounding medium. In the present work, to tune their LSPR properties, a series of Al NCs with their average sizes (nm) of 25.3 ± 4.9, 29.8 ± 5.4, 35.1 ± 7.5, 44.9 ± 12.4, 56.2 ± 18.1, 75.5 ± 22.3, and 98.6 ± 23.4 were synthesized with the Ti(i-PrO)4 concentrations of 560, 300, 200, 112, 56, 36, and 16 μM, respectively. To characterize the LSPR properties of the as-synthesized Al NCs, we measured the extinction spectra of Al NCs in THF solution without exposure to air ( Supporting Information Figure S4). Due to the cutoff wavelength of THF at 270 nm, the extinction peaks of small Al NCs could not be fully detected. To overcome the limitation of the THF, we transferred PEO-decorated Al NCs into deionized water with a cutoff wavelength of < 200 nm. Figure 2a shows the UV–vis extinction spectra of the Al NCs with different sizes in freshly prepared aqueous solutions. The LSPR peaks were redshifted continuously, as their sizes became larger with reducing Ti(i-PrO)4 concentrations during synthesis. This result indicated that the LSPR properties of the Al NCs could be tuned by adjusting the concentration of the Ti(i-PrO)4 catalyst. With the following relatively smaller D values (nm): 25.3 ± 4.9, 29.8 ± 5.4, and 35.1 ± 7.5, the Al NCs possessed relatively sharp LSPR peaks with FWHM as small as 80, 99, and 103 nm, respectively. The LSPR peaks were redshifted from 250 to 300 nm as the size increased. Further increasing the Al NC size to larger D (44.9 ± 12.4, 56.2 ± 18.1, 75.5 ± 22.3, and 98.6 ± 23.4 nm), also resulted in progressive redshift of the LSPR peaks, but became much broader. Meanwhile, a new peak with shorter wavelength appeared, and also, redshifted from 209 to 234 nm, as the size of Al NCs increased. To understand the LSPR properties of the prepared Al NCs, we investigated systematically, the shape-specific plasmonic properties of different-size Al NCs using finite difference time domain (FDTD) simulation (Figures 2c–h), whereby, all the Al NCs were assumed to be pure metallic core without an oxide layer (the influence of oxide layer and sharp corner of 25, 30, and 100 nm Al NCs are discussed in Supporting Information Figures S5 and S6). For each size, the calculated extinction spectra of Al NCs (red line) were the sum of two components, that is, the extinction spectra of the prism- (blue line) and octahedron- (green line) shaped NCs, based on their number fractions, determined by TEM analysis. For small Al NCs with average D of 25 and 30 nm, truncated prism and octahedron corresponding to the morphologies in the TEM images (Figure 2c and d) were modeled. We considered the random orientation of the prism and octahedron in solution and used orientationally averaging to produce their ensemble spectra (See Supporting Information Figure S7). We found that the extinction spectra of the octahedron-shaped Al NCs showed one strong dipole resonance, which redshifted from 236 to 351 nm, as the D increased from 25 to 100 nm. Relative weak quadrupole resonance bands with a wavelength of 200 nm were observed for large NCs with D of 80 and 100 nm. The line shapes of simulated spectra agreed well with those of Au octahedron of similar sizes, attributable to the structural features of the octahedron.45–47 In contrast, the simulated extinction spectra of prism-shaped Al NCs showed both dipole and quadrupole resonance bands for all the sizes. With the increase of NC size, both bands redshifted; more importantly, the separation between the two bands increased significantly. This observation was also in agreement with size-tunable plasmon bands of Au prisms.48–51 The large separation of dipolar and quadrupolar resonance bands for the octahedron and the mismatch between the resonances of prism and octahedron caused the peak broadening for the large NCs. The simulated extinction spectra for all the different Al NC sizes showed a good agreement with those of experimental ones (black dot line). Discrepancies between the simulated and experimental Al NCs could be attributed to their additional shapes and their broad size distribution in the ensemble solutions. In comparison to noble metal nanoparticles such as silver and gold, sub-100 nm Al NCs displayed specific size-tunable LSPR properties at lower wavelengths (λ < 400 nm).47–51 The molecular weight of the PEO played an important role in controlling the synthesis of Al NCs. Previously, Xia's group52 reported that the coverage density of polyvinylpyrrolidone on Ag NC surfaces played a critical role in determining the surface free energies (γ) of Ag facet planes, {100} and {111}, and thus, the shapes of Ag NCs. Low-molecular-weight polyvinylpyrrolidone (PVP) is more effective in reducing the free energy of Ag {100} than high-molecular-weight ones.52 To find out the role of the molecular weight in the present system, we synthesized Al NCs under similar conditions (e.g., the precursor concentration, temperature, mass concentrations (Cm) of PEO, using a series of varying catalyst concentrations), except for the use of PEO-2 k, PEO-10 k, and PEO-100 k with Mn of 2.0, 10.0, and 100 kg/mol, respectively (Figures 3a–c; Supporting Information Figure S10). Although the size of Al NCs was tuned more effectively by adjusting the catalyst concentration, the D ranges of the Al NCs were distinct for PEO with three different molecular weights. PEO-2 k was more suitable for synthesizing small Al NCs with a D range of ∼ 27 to 70 nm. When the D was larger than 70 nm, the Al NCs became quite polydisperse with coalescence and aggregation. This result suggested that the short polymer chain could not prevent the aggregation of Al NCs during their growth, consistent with our previous observations.42 For the highest molecular weight (Mn of 100 kg/mol), the D range of Al NCs increased slightly from 40 to 90 nm but with broader size distributions (SD >30%). The viscosity of the reaction solution for PEO-100 k increased significantly, and therefore, reduced the diffusion and reaction rate. The larger size, and thus, the steric effect of high-molecular-weight PEO also make it harder for the polymer chains to adsorb and pack on the surface of Al NCs, resulting in less control of their growth and size. We found PEO-10 k is the optimal molecular weight for the synthesis of Al NCs, yielding the widest D range from 35 to 150 nm with SD of ~15%. PEO-10 k does not lead to a viscous solution as PEO-100 k does, while it is large enough for avoiding the aggregation and coalescence of Al NCs. Therefore, PEO-10 k is capable of controlling the growth of a wide D range of Al NCs. Figure 3 | The size-control ability of different molecular weight (Mn) PEO in synthesizing Al NCs at various mass concentrations (Cm = 4, 6.5, 9, 11.5 mg/mL). (a) PEO-2 kg/mol. (b) PEO-10 kg/mol. (c) PEO-100 kg/mol. (d) Comparing the D range synthesized by 2, 10, 100 kg/mol PEO at 11.5 mg/mL. Download figure Download PowerPoint To study the interaction between PEO and the surface of Al NCs,53 we compared the Fourier transform-infrared (FTIR) spectra of PEO on Al NCs and pure PEO (Figure 4; Supporting Information Figure S8). As shown in Figure 4 and listed in Table 1, the stretching vibration (Vs) and deformation vibration (Vd) peaks of ether oxygen bonds (C–O–C) in PEO for the synthesis of Al NCs were blueshifted from 1110 to 1101 cm−1 and redshifted from 1060 to 1068 cm−1, respectively. This result confirmed the binding interaction between the C–O–C groups and Al atoms on the NC surface.54 Meanwhile, the bending vibration (Vb) peak of methylene (–CH2–) of PEO on the Al NC surface was broadened and blueshifted from 1467 to 1457 cm−1.55 The wagging vibration (Vw) peak of methylene (–CH2–) was also redshifted from 1360 to 1376 cm−1. The new peak observed at 900 cm−1 in PEO-synthesized Al NC was assigned to the Al–O bond.56 Figure 4 | FTIR spectra of PEO (green) and Al [email protected] (blue). Download figure Download PowerPoint Table 1 | Detailed Wavenumber Shift of FTIR spectra Vb (–CH2–) Vw (–CH2–) Vs in plane (C–O–C) Vas (C–O–C) PEO 1467 cm−1 1360 cm−1 1280 cm−1 1236 cm−1 Al [email protected] 1457 cm−1 1376 cm−1 1267 cm−1 1231 cm−1 Vss (C–O–C) Vs (C–O–C) Vd (C–O–C) V (Al–O) PEO 1145 cm−1 1110 cm−1 1060 cm−1 – Al [email protected] 1150 cm−1 1101 cm−1 1068 cm−1 900 cm−1 To gain a deeper understanding of how the chemical structure of PEO made it a suitable ligand for the synthesis of Al NCs, we compared the effectiveness of using other four ligands, namely, 18-Crown-6 ether, polypropylene oxide (PPO), Pluronic F127, and polycaprolactone (PCL). The information on the selected ligands is listed in Table 2. Table 2 | Chemical Stability and Size-Control Ability of Selected Ligands Ligand Number of Repeat Units Chemical Stabilitya Size-Control Abilityb 18-Crown-6 6 Stable Poor PPO 51 Stable Ordinary F127 200/PEO+65/PPO Stable Good PCL 700 Decompose – aReacted with H3Al(1-MP) at 50 °C for 30 min: stable for no obvious changes in 1 H NMR and/or GPC measurement. bGood, ordinary, and poor for the size standard deviations of Al NCs for <20, <50, and ≥50%, respectively. The 18-Crown-6 could be viewed as a small ring-like PEO with six ether oxygen in each molecule. The Al NCs synthesized with 18-Crown-6 in (Figure 5a) showed cuboctahedron and some irregular shapes but with a much broader D range from several hundreds to tens of nanometers. This result indicated that although 18-Crown-6 could absorb on certain Al facets, it possessed extremely limited control on the size of Al absorption, and provided not enough steric hindrance to regulate the growth of NCs. PPO could be considered as a PEO with an additional methyl group in the repeat unit. The Al NCs synthesized in the presence of PPO (Figure 5b) contained a large fraction of small octahedrons and prisms (<50 nm), accompanied by large, irregular-shaped NCs (>100 nm). In comparison with PEO, the weaker size and shape control of PPO might have been caused by the steric hindrance effect of its methyl groups besides the ether oxygen, which has a less affinity to Al atoms.57 On the contrary, the ether bond in the PEO chain could coordinate readily with the Al surface.58 As shown in Figure 5c, the Al NCs synthesized with F127 contained regular octahedrons and prisms with much better uniformity (average D of 52.3 ± 10.0 nm), which were similar to those synthesized with PEO. This result suggested that the PPO block did not influence the facet absorption capacity of the PEO blocks in F127. Furthermore, we used PCL to verify if the additional oxygen in an ester bond could provide stronger coordination of the Al surface. However, the ester bond in PCL reacted with H3Al, resulting in a gelatinous solution and no NCs formation. To test their chemical stabilities, the THF solutions of PCL and PEO (Figures 5d and 5e) were heated to 50 °C and reacted with H3Al precursor for 30 min.59 After the reaction, the THF solution of PCL became cloudy, yielding white gel-like precipitates, marked by the yellow circles in Figure 5d. Meanwhile, the reaction solution of PEO remained transparent, and no precipitates were observed (Figure 5e). Then we used proton nuclear magnetic resonance (1H NMR) and gel permeation chromatography (GPC) to investigate the chemical stability of PCL and PEO. The results shown in Supporting Information Figure S9 reveal that the PEO did not change, both in its chemical shifts of the ethyl group in NMR or its retention time in GPC. In contrast, the PCL decomposed to oligomers with Mn below the detection limit (2.0 kg/mol) of GPC. Collectively, these results demonstrated that PEO exhibited good chemical stability with H3Al precursor during the synthesis of Al NCs; its polydentate structure with the repetitive ether–oxygen binding on Al NC surfaces was critical for controlled synthesis of Al NCs.60 Figure 5 | (a–c) Al NCs synthesized by (a) 18-Crown-6 ether, (b) polypropylene oxide (PPO), and (c) Pluronic F127 as surface ligands. (d–e) The solutions of (d) PCL and (e) PEO-10 k reacted with H3Al(1-MP) at 50 °C for 30 min, respectively. The yellow circles in (d) mark the white gel-like precipitates in the PCL solution. Download figure Download PowerPoint The stability of Al NCs is critical for their practical applications.61 As mentioned earlier, upon exposure to air, a self-limiting oxide layer formed on the surface of the Al NCs. This passivation layer could prevent the metal Al from further oxidation effectively. After storing the Al NCs with the oxide layer in THF for 4 months under ambient condition (room temperature and under air atmosphere), their LSPR peak positions showed no obvious changes, compared with those before ( Supporting Information Figure S11). Meanwhile, the intensity of the band at longer wavelength was slightly increased, probably due to the slight aggregation during the storage. This result indicated that the passivation layer could prevent further oxidation by O2 even for Al NCs with larger surface-to-volume ratios, compared with the bulk Al. Further, these Al NCs were transferred into deionized (DI) water to test their stability with H2O. We used UV–vis spectroscopy to monitor the LSPR change of the Al NCs during their reaction with DI water. As shown in Figure 6, the LSPR spectra obtained for each sample at 1 and 5 min were overlapped totally, suggesting that the Al NCs were very stable over short periods. For long reaction time ranging from 1.5 to 66 h, the LSPR peaks were redshifted initially, and then blueshifted, accompanied by continuous decrease in their intensities and broadening of their FWHM (Figures 6a–e; Supporting Information Figure S12). Meanwhile, the line shape of LSPR curves seldom changed, even after reaction for 66 h. The TEM images of Al NCs after reaction with DI water for 40 h were obtained (Figures 6j–m). The result revealed that the Al NCs show a clear core–shell structure, in which the thickness of the outer oxide/hydroxide layer reached ∼ 8.5 nm, compared with the 3.5 nm obtained for the oxide layer before the reaction with DI water. These changes in the dielectric constant and thickness of the outer layer could lead to the initial redshift observed with the LSPR peaks of Al NCs,16 while the shrink of metallic core sizes might have caused their blueshift. More importantly, after the DI water reaction, the shape of the metallic core was almost preserved, compared with the original Al NCs in this study, no matter their initial size ranges. This result could be attributable to the nearly constant LSPR line shape of the Al NCs before and after reaction with DI water. Figure 6 | The stability of Al NCs in aqueous solution. (a–e) LSPR evolution of different-sized Al NCs during reacting with deionized water. (f–i) Representative TEM images of different-sized Al NCs after stored in THF for four months under ambient condition. (j–m) Representative TEM images of different-sized Al NCs after reacted with deionized water for 40 h. The scale bars in (f–m) are equal to 20 nm. Download figure Download PowerPoint Conclusion We have developed a simp