This document is the unedited Author’s version of a Submitted Work that was subsequently accepted for publication in Chemistry of Materials, copyright © American Chemical Society after peer review. To access the final edited and published work see: https://dx.doi.org/10.1021/acs.chemmater.9b02005. Chemistry of Materials This document is confidential and is proprietary to the American Chemical Society and its authors. Do not copy or disclose without written permission. If you have received this item in error, notify the sender and delete all copies. Seeded-growth Aqueous Synthesis of Colloidal-Stable Citrate-Stabilized Au/CeO2 Hybrid Nanocrystals: Heterodimers, Core@Shell, Clover- and Star-like Structures. Journal: Chemistry of Materials Manuscript ID cm-2019-02005z.R2 Manuscript Type: Article Date Submitted by the 26-Aug-2019 Author: Complete List of Authors: Piella, Jordi; Institut Català de Nanociència i Nanotecnologia, Gonzalez-Febles, Ana; Institut Català de Nanociència i Nanotecnologia Patarroyo, Javier; Institut Català de Nanociència i Nanotecnologia, Arbiol, Jordi; ICREA and Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology (BIST), Advanced Electron Nanoscopy Bastús, Neus; Institut Català de Nanociència i Nanotecnologia, Puntes, Víctor; Institut Catala de Nanotecnologia, Inorganic Nanoparticles Group ACS Paragon Plus Environment Page 1 of 19 Chemistry of Materials 1 2 3 4 Seeded-growth Aqueous Synthesis of Colloidal-Stable Citrate- 5 Stabilized Au/CeO2 Hybrid Nanocrystals: Heterodimers, 6 7 Core@Shell, Clover- and Star-like Structures. 8 9 10 Jordi Piella1, Ana Gónzalez-Febles1, Javier Patarroyo1, Jordi Arbiol1, 2, Neus G. Bastús1 *, 11 12 and Víctor Puntes1, 2, 3 * 13 14 1 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC & BIST, Campus UAB, 15 Bellaterra, Barcelona, Spain. 16 2 ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Catalonia, Spain 17 18 3 Vall d’Hebron Institut de Recerca (VHIR), 08035, Barcelona, Spain. 19 20 21 22 * To whom correspondence should be addressed: neus.bastus@icn2.cat, victor.puntes@icn2.cat 23 24 25 26 ABSTRACT 27 28 Well-defined colloidal-stable citrate-stabilized Au/CeO2 hybrid nanocrystals (NCs) with coherent 29 30 quasi-epitaxial interfaces and unprecedented control of their architectural and morphological 31 characteristics have been synthesized via a novel and straightforward seeded-growth aqueous 32 approach. The synthetic strategy, based on the identification of the experimental conditions 33 34 under which the heterogeneous nucleation and growth processes of CeO2 onto pre-synthesized 35 Au is controlled, allows for the fine adjustment of each individual domain in the structure, 36 particularly, the size of the Au core (from 5 to 100 nm), the thickness of the CeO2 shell (from 5 to 37 38 20 nm) and the growth mode of CeO2 onto Au NCs (from core@shell to heterodimer, clover- and 39 star-like structures). This morphological control is achieved by the rational use of sodium citrate 40 which plays multiple key roles, as a reducer and stabilizing agent in the preparation of the Au 41 NCs, and as a complexing agent of Ce3+ for its controlled oxidation and hydrolysis during the 42 43 subsequent CeO2 deposition. The resultant Au/CeO2 NCs remain stable and well-dispersed in 44 water, allowing to study the impact of fine variations of NC structure on the underlying optical 45 response. This level of morphological control, as well as the ease by which such well-defined 46 47 nanostructures are produced, opens new opportunities for systematically investigate the 48 interactions between individual components in designing more advanced complex NCs. 49 Remarkably, because no organic solvents are used and no toxic waste is formed during the 50 reaction, the proposed synthesis method can be defined as a sustainable, viable and low-cost 51 52 effective. 53 54 55 56 57 58 59 60 1 ACS Paragon Plus Environment Chemistry of Materials Page 2 of 19 1 2 3 INTRODUCTION 4 5 The rational design and development of new protocols for the colloidal synthesis of 6 7 multicomponent nanocrystals (NCs) represent an important research direction to expand the 8 9 functionalities of single-component counterparts.1-3 In these systems, the functionality of the 10 composite NCs is ultimately determined by the atomic interactions between constituent domains, 11 12 which essentially relies on the precise and deliberate control of the NC architecture with the 13 14 independent organization of each individual domain.4 Thus, while in heterodimer conformation 15 16 the accessible reactive sites at the interface enhance the catalytic performance of NCs5-8, in 17 core@shell geometries the interaction and electronic transfer between both domains are 18 19 maximized, offering great applicability in plasmonics and sensing.9-11 20 21 Among the variety of developed materials, the controllable integration of noble metal and metal 22 23 oxides into single NCs has attracted significant interest due to their unique structural features 24 and physicochemical properties.12-13 Despite advances in the field, the rational development of 25 26 synthetic strategies enabling the precise control over the architecture of noble metal/metal oxide 27 28 NCs and their interface still represents a difficult task, especially in water, where the 29 30 identification of the relevant synthetic parameters governing their final morphology is a no 31 solved issue. The deposition of a crystalline overlayer onto a crystalline substrate implies dealing 32 33 with the atomic geometrical constraints of the materials involved, which is challenging for 34 35 materials with large lattice mismatch. A paradigmatic case is Au/CeO2 NCs. These materials are 36 of interest because they have shown advanced optical properties and excellent catalytic activity 37 38 due to the combination of the outstanding oxygen storage properties of CeO2 facilitated by the 39 40 metallic electron cloud of Au.14-15 41 42 Traditionally, Au/CeO2 composites were easily prepared by impregnation and co-precipitation 43 44 methods. The co-precipitation method takes advantage of the very low solubility of CeO2 at 45 alkaline pH, and because of its simplicity is the method of choice for industrial production, 46 47 although the morphological control and stability of the resulting material is extremely poor.16-18 48 49 Alternatively, colloidal chemistry strategies have allowed a much-improved control of material 50 morphology.19 Among them, one-pot methods based on the simultaneous oxidation and 51 52 reduction of the precursors (Au3+ to Au0 each 3 Ce3+ to Ce4+), provide an easy way to force their 53 54 precipitation into a single NC.20-23 However, since Ce3+ oxidation and hydrolysis are promoted at 55 alkaline pH and the reactivity of Au3+ in these conditions is very low24, the redox process is not 56 57 spontaneously favored, restricting the reaction conditions and thus hampering the architectural 58 59 and morphological control of the system. 60 2 ACS Paragon Plus Environment Page 3 of 19 Chemistry of Materials 1 2 3 Alternatively, seed-mediated strategies have the advantage of decoupling Ce3+ oxidation from 4 5 Au3+ reduction allowing to control these processes independently. However, the large lattice 6 7 mismatch between CeO2 (0.5412 nm) and Au (0.4065 nm), and the strong preference of cerium 8 precursors to homonucleate during condensation rather than to heteronucleate makes the 9 10 controlled deposition of CeO2 at the surface of Au NCs difficult. In this regard, the complexation 11 12 of Ce precursors with strong affinity ligands may provide a much lower hydrolysis rate which 13 helps to avoid the homogeneous nucleation of CeO2.25 14 15 Strategies exploiting the use of functional ligands to direct the deposition of CeO2 onto pre- 16 17 synthesized metal NCs are an interesting approach with increasing popularity.25-28 Following this 18 19 idea, ligands drive the absorption of Ce3+ and Ce4+ onto the metal surface that, upon heating, 20 21 decomposes forming the hybrid structure. Stated examples include the use of EDTA,25 CTAB,25 22 glucose,26 and mercaptocarboxilic acids.27-28 Although interesting, these strategies are 23 24 considerably complicated, often requiring multiple steps, including washing and 25 26 functionalization of the pre-synthesized metal NCs, and post-thermal and –calcination treatments 27 for the elimination of the ligands that may lead to unwanted NC transformations (sintering and 28 29 aggregation).21, 29-30 Additionally, the use of ligands with strong affinities for metal surface blocks 30 31 further surface accessibility. 32 33 Herein, we report a general, straightforward and novel seeded-growth approach for the 34 35 preparation of stable colloidal solutions of monodisperse citrate-stabilized Au/CeO2 NCs with 36 coherent and quasi-epitaxial interfaces. The method allows a fine adjustment of each individual 37 38 domain in the structure and explicitly different coupling manners between the Au and CeO2 39 40 counterparts. Particularly, the size of the Au core, the thickness of the CeO2 shell and the growth 41 mode of CeO2 onto Au NCs can be manipulated in an independent, rational, and systematic way. 42 43 The strategy relies on the use of sodium citrate (SC) as a unique co-reagent. SC is a well-known 44 45 non-toxic and biocompatible substance widely-used in the synthesis of noble metal NCs.31-32 In 46 this work, we extend its use to oxide systems by employing its ability to act as a complexing agent 47 48 of Ce3+/4+ ions, thereby adjusting their oxidation and hydrolysis rates in water. Interestingly, 49 50 because this strongly depends on the reaction pH, it is by adjusting this parameter that 51 architectures varying from core@shell to clover-like and heterodimers can be obtained. In 52 53 addition, Au can be overgrown onto clover-like Au@CeO2 NCs, yielding to tip-free CeO2-coated 54 55 Au star-like NCs. Different morphologies entail different properties. Thus, while heterodimer 56 morphologies offer accessibility to both domains, core@shell structures are more attractive when 57 58 seeking electronic confinement. 59 60 3 ACS Paragon Plus Environment Chemistry of Materials Page 4 of 19 1 2 3 The overall procedure is simple, highly reproducible and avoids time-consuming processes 4 5 because it does not require calcination or any other complicated steps. Furthermore, resultant 6 7 Au/CeO2 NCs remain stable and well-dispersed in water, and show bright colors with well- 8 defined surface plasmon resonances, allowing studying the impact of fine variations of hybrid 9 10 nanostructure on the underlying optical response. 11 12 RESULTS and DISCUSSION 13 14 15 In order to control the nucleation and growth of CeO2 exclusively onto previously synthesized 16 Au NCs, our strategy is to provide a lower reactivity of Ce3+ precursor in water by its 17 18 complexation with sodium citrate (SC).25, 33 Thus, for the synthesis of core@shell Au@CeO2 hybrid 19 20 NCs, citrate-stabilized Au seeds of ~30 nm are first prepared as previously described31 and used 21 without further purification. The formation of the CeO2 shell is achieved by injecting an aqueous 22 23 solution of Ce(NO3)3 into a boiling solution of SC containing a specific amount of the Au seeds. 24 25 The citrate rapidly complex and stabilize the Ce3+ ions that, upon boiling at alkaline conditions, 26 slowly oxidize -by dissolved O2-, hydrolyze and deposit at the metal surface, leading to Au@CeO2 27 28 NCs. This process results in a progressive change in the color of the reaction mixture from red- 29 30 wine (Au seeds) to bright/intense purple (Au@CeO2 NCs), and takes about 4 hours to complete. 31 32 Representative images of transmission electron microscopy (TEM) and high-angle annular dark 33 34 field scanning TEM (HAADF-STEM) of as-obtained colloids (Fig. 1A-D) show the systematic 35 36 formation of highly monodisperse Au@CeO2 NCs consisting of an electron-dense core of Au (~30 37 38 nm) surrounded by a relatively uniform crystalline CeO2 shell (~7 nm). Interestingly, no isolated 39 CeO2 were observed in the final product, indicating the controlled growth of CeO2 onto the Au 40 41 NC surface. According to HRTEM images (Fig. 1E, S1-2), the shell is composed of tiny CeO2 NCs 42 43 with sizes between 2-3 nm, assembled together around the Au core with coherent quasi-epitaxial 44 interfaces. Details of the squared region and its corresponding power spectrum reveal that the 45 46 individual CeO2 NCs are crystalline with lattice space corresponding well with the characteristic 47 48 (111), (002) and (220) planes of fluorite phase of CeO2. These results are further supported by the 49 XRD analyses (Fig. S3). Finally, elemental chemical composition maps obtained by electron 50 51 energy loss spectroscopy (EELS) (Fig. 1F) of selected Au@CeO2 NCs (Fig. 1D) corroborate the 52 53 encapsulation of the Au core by the CeO2 NCs. 54 55 56 57 58 59 60 4 ACS Paragon Plus Environment Page 5 of 19 Chemistry of Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 Figure 1. Synthesis of Au@CeO2 Hybrid Nanocrystals. Representative TEM images (A-C) and 18 HAADF-STEM images (D) of as-obtained Au@CeO2 NCs revealing the formation of highly 19 monodispersed hybrid nanostructures consisting on an electron-dense core of Au (~30 nm) 20 surrounded by a relatively uniform crystalline CeO2 shell (~7 nm). HRTEM images (E) reveal the 21 details of the CeO2 shell, which is composed of small NCs with sizes of 2-3 nm assembled together 22 around the Au core. Detail of the red squared CeO2 region and its corresponding electron 23 diffraction pattern show the f.c.c structure of the CeO2 phase. Electron energy loss spectroscopy 24 25 (EELS) elemental maps (F) obtained from selected hybrid NCs (D) confirms the core@shell 26 structure. Additional HRTEM characterization of these NCs is shown in Fig. S1. 27 28 29 30 One of the main advantages of adopting a seeded-growth approach is the possibility to precisely 31 32 and independently tailor the Au core and the CeO2 shell dimensions.31, 34 Thus, well-defined 33 34 core@shell Au@CeO2 NCs with different core sizes were obtained by using Au NC seeds with 35 36 diameters of 6.5, 10, 15, 30, 40 and 100 nm (Fig. 2, Table 1). The impact of the CeO2 coating in the 37 localized surface plasmon resonance (LSPR) of these Au@CeO2 NCs was studied by UV-Vis 38 39 spectroscopy. The presence of the CeO2 coating results in both an increase in the LSPR peak 40 41 intensity of the Au core and a systematic red-shift of its position. The great shift, as large as 86 42 nm in the case of 100 nm Au cores with a CeO2 shell of 18 nm, along with the increased absorbance 43 44 of the samples, are in agreement with the high refractive index of the CeO2 coating layer.20, 25 45 46 Remarkably, the full width at half-maximum (FWMH) of the plasmon band is well-maintained, 47 proving not only that the morphology of the initial Au seeds is preserved after the CeO2 coating 48 49 but also that the samples remain well-dispersed in solution. These changes in the optical 50 51 properties upon the formation of the CeO2 shell also result in color changes of the samples: from 52 brown-red to pink-red (small Au cores, Fig. 2H), from red to purple (medium Au cores, Fig. 2J) 53 54 and finally from red to blue (large Au cores, Fig. 2L). 55 56 57 58 59 60 5 ACS Paragon Plus Environment Chemistry of Materials Page 6 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 Figure 2. Size control of Au core in the synthesis of Au@CeO2 Hybrid Nanocrystals. 54 Representative TEM, SEM and HAADF-STEM images of well-defined core@shell Au@CeO2 NCs 55 with different core diameters, and corresponding UV–vis spectra before and after formation of 56 57 the CeO2 shell onto Au seeds of 6.5 nm (A, B), 10 nm (C, D), 15 nm (E, F), 32 nm (G, H), 40 nm (I, 58 J) and 98 nm (K, L). In all cases, the LSPR peak of the initial Au seeds (black line) increases in 59 intensity and red-shifts after the CeO2 coating (blue line). Samples are detailed in Table 1. 60 6 ACS Paragon Plus Environment Page 7 of 19 Chemistry of Materials 1 2 3 Table 1. Tested conditions for the preparation of Au@CeO2 NCs with controlled Au core size. 4 5 SYNTHESIS CONDITIONS TEM SIZE LSPR POSITION 6 SAMPLE Au seeds [Au seeds] Ce(NO3)3 Au core CeO2 shell Au Au@CeO2 Δλ 7 diam. (nm) (NP/mL)* (mL)** diam. (nm) (nm) λ (nm) λ (nm) (nm) 8 FIG.2A 6.5 ± 0.6 ~1.1 1013 1.5 6.8 ± 1.1 4.0 ± 0.7 513 535 22 9 FIG.2C 9.9 ± 0.8 ~2.5 1012 1.5 9.7 ± 1.5 6.6 ± 1.2 517 547 30 10 FIG.2E 14.6 ± 1.3 ~7.8 10 11 1 14.8 ± 1.8 7.2 ± 1.2 519 549 30 11 FIG.2G 32.0 ± 2.6 ~1.3 1011 0.6 31.9 ± 2.6 6.6 ± 0.9 525 550 25 12 FIG.2I 40.1 ± 3.8 ~6.8 1010 0.6 40.3 ± 3.9 8.3 ± 0.9 530 560 26 13 FIG.2K 98.3 ± 16.0 ~5.9 109 0.75 98.5 ± 15.6 18.4 ± 4.3 571 616 45 14 *100 mL of a citrate solution (5 mM) with adjusted pH 9-10. **Specific volume of a 25 mM solution. 15 16 17 The method also allows the fine adjustment of the amount of CeO2 deposited onto the Au cores 18 19 by simply adjusting the ratio between Au seeds to Ce3+ precursor (Fig. 3, Table 2). Thus, the 20 addition of low amounts of Ce3+, or alternatively the increased number of seed NCs present in 21 22 solution, leads to the formation of non-uniform clover-like structures with uncompleted shell 23 24 morphologies (Fig. 3A, D) that become complete core@shell structures when the amount of Ce3+ 25 was increased (Fig. 3B-C, D-F). As a result, clover-like Au@CeO2 NCs, where parts of the Au 26 27 surface are still exposed, experience smaller red-shifts than NCs with a uniform and thick CeO2 28 29 coating around the Au cores (Fig. 3G, H).35 A further increase of injected Ce3+, required for the 30 31 formation of thicker CeO2 shells (>20 nm) in a single step, results in the nucleation of separate 32 CeO2 NCs. In this case, the sequential and slow injections of Ce3+ precursor prevents the system 33 34 from self-nucleation,31 thereby leading to the formation of thicker CeO2 shells. Note that the 35 36 possibility to adjust the thickness and dimensionality of the CeO2 shell is important to design the 37 final applicability of the NC. Thus, while the thinner and low compact shell in the clover-like 38 39 structures maximize the exposed interface between both materials and provides a shorter 40 41 diffusion pathway for reactants to reach the Au core, the thicker and uniform shell endows the 42 NC with long-term stability against chemical transformations (dissolution and aggregation).36 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 7 ACS Paragon Plus Environment Chemistry of Materials Page 8 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 Figure 3. Control of CeO2 shell thickness in the synthesis of Au@CeO2 Hybrid NCs. 39 40 Representative TEM and HAADF-STEM images of core@shell Au@CeO2 NCs with increasing 41 CeO2 shell thickness (A-F): Au cores of 15 nm with increasing CeO2 shell from 5.3 nm (A), 7.2 nm 42 (B) and 10.5 nm (C), and Au cores of 32 nm with increasing CeO2 shell from 6.6 (D), 8.5 (E) and 43 10.3 nm (F). UV-vis spectra of the corresponding Au colloids before and after CeO2 shell 44 formation (G, H) reveals a strong dependence between the LSPR peak (intensity and position) 45 and the thickness of the CeO2 shell. Synthesis conditions are detailed in Table 2. 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 8 ACS Paragon Plus Environment Page 9 of 19 Chemistry of Materials 1 2 3 Table 2. Tested conditions for the preparation of Au@CeO2 NCs with controlled CeO2 shell 4 thickness. 5 6 SYNTHESIS CONDITIONS TEM SIZE LSPR POSITION 7 SAMPLE Au seeds [Au seeds] Ce(NO3)3 Au core CeO2 shell Au Au@CeO2 Δλ 8 9 diam. (nm) (NP/mL)* (mL)** diam. (nm) (nm) λ (nm) λ (nm) (nm) 10 FIG.2A 14.6 ± 1.3 ~7.8 1011 0.75 14.8 ± 1.6 5.3 ± 1.1 519 542 23 11 FIG.2C 14.6 ± 1.3 ~7.8 1011 1 14.8 ± 1.8 7.2 ± 1.2 519 549 30 12 FIG.2E 14.6 ± 1.3 ~7.8 1011 2.5 15.7 ± 1.8 10.5 ± 1.8 519 556 37 13 FIG.2G 32.0 ± 2.6 ~1.3 1011 0.6 31.9 ± 2.6 6.6 ± 0.9 525 550 25 14 FIG.2I 32.0 ± 2.6 ~1.3 1011 1 32.1 ± 2.6 8.5 ± 0.9 525 557 32 15 FIG.2K 32.0 ± 2.6 ~1.3 10 11 1.5 32.8 ± 2.9 10.3 ± 1.5 525 560 35 16 *100 mL of a citrate solution (5 mM) with adjusted pH 9-10. **Specific volume of a 25 mM solution. 17 18 The controlled deposition of CeO2 domains onto the surface of Au seeds is promoted by the use 19 20 of SC, which plays multiple roles (Scheme 1). Note that citrate already acts as a reducer and 21 22 stabilizer in the synthesis of the Au seeds, providing electrostatic repulsion to the resultant Au 23 NCs (role#1 and #2).31 Next, in the formation process of the CeO2 shell, citrate efficiently complex 24 25 Ce3+/4+ leading to a precursor with a much lower oxidation and hydrolysis rates (role#3) (see SI).33 26 27 In a control experiment (Fig. S5), the lack of SC led to the formation of isolated and strongly 28 29 aggregated CeO2 NCs, indicating how the greater stability of the citrate-complexed Ce3+/4+ 30 precursor favours the formation of Au@CeO2 NCs. Also, the employed SC solution maintains the 31 32 reaction mixture at mild alkaline conditions, thereby promoting the oxidation of Ce3+ to Ce4+ by 33 34 the dissolved O2 but not its precipitation in the form of Ce(OH)3, a step often required in the 35 formation of the CeO2 shell (role#4).29 Note that in the absence of oxygen (in deoxygenated water), 36 37 neither oxidation of Ce3+ nor deposition of a Ce(OH)3 shell were observed as control experiments 38 39 show (Fig. S6). Finally, the well-known affinity of carboxylic groups of citrate for metal and metal 40 oxide surfaces ensures the colloidal stability of the resultant Au@CeO2 NCs25, 27-28 (role#5), as zeta 41 42 potential measurements reveal (Fig. S7). Remarkably, this interaction with citrate does not 43 44 prevent further functionalization of the NCs with the molecules of interest (Fig. S8). 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 9 ACS Paragon Plus Environment Chemistry of Materials Page 10 of 19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 Scheme 1. Roles of SC in the Synthesis of Au@CeO2 Hybrid NCs. Citrate acts as a reducer and 21 stabilizer in the synthesis of Au seeds. Then, during the formation process of CeO2, it effectively 22 complex Ce3+ and Ce4+ ions, preventing their spontaneous hydrolysis and precipitation in water. 23 The buffer effect of sodium citrate also prevents the solution pH from dropping towards acidic 24 values during the synthesis and favours the oxidation of Ce3+ to Ce4+. Finally, citrate adsorbs on 25 26 the CeO2 surface through their carboxylic groups and keeps the Au@CeO2 NCs well-dispersed in 27 solution. 28 29 30 31 Because the complexation of Ce3+ and Ce4+ with citrate forming intermediate species is expected 32 33 to be strongly dependent on the pH, the effect of this parameter on the reaction kinetics and final 34 35 NC structure was evaluated. By performing the reaction at acidic pH 5.5, partially substituting 36 SC by citric acid, no deposition of CeO2 onto the Au seeds was observed (Fig. S9A). Under these 37 38 conditions, Ce3+ did not oxidize nor hydrolyze, even in the presence of a metal surface. When the 39 40 pH was set at nearly neutral values (pH 7-8), the reaction proceeded very slow (5h) (Fig. S10A) 41 42 and resultant NCs were heterodimers composed by two well-defined compositional domains 43 (Fig. 4A-B, S11). As pH was increased (pH 9-10), the reaction progressed faster (1-2h) (Fig. S10B) 44 45 and the obtained Au@CeO2 NCs were core@shell structures (Fig. 4C). Finally, the further increase 46 47 of the pH towards highly alkaline values (pH 12) resulted in the fast and spontaneous nucleation 48 of isolated CeO2 NCs away from the surface of the Au seeds (Fig. S9B). 49 50 51 Obtained results show how the use of citrate as a complexing agent of Ce3+/4+ facilitated control 52 53 over the oxidation and hydrolysis rates of cerium precursors, reducing their reactivity in water 54 and allowing for a better control of the reaction. In these conditions, the different growth modes 55 56 of CeO2 can be adjusted by simply varying the reaction pH. This process determines the tendency 57 58 of the cerium precursor to (i) homogeneously nucleate (high pH), (ii) hetero-nucleate at the metal 59 surface (mild alkaline), (iii) growth (neutral) or (iv) remain stable in solution (acidic). As stated 60 10 ACS Paragon Plus Environment Page 11 of 19 Chemistry of Materials 1 2 3 previously37-38, hydroxide ions in solution are highly involved in the formation process of CeO2 4 5 NCs. By decreasing the concentration of hydroxide ions, the oxidation and hydrolysis rates of 6 7 the complexed cerium ions are further decreased39-40, which is translated into even slower reaction 8 rates. Under these conditions, the homogeneous nucleation of CeO2 is prevented while its 9 10 heterogeneous nucleation onto the Au seeds is promoted. Then, once a small domain of CeO2 11 12 NCs is initially formed on the Au seeds, the subsequent CeO2 NCs prefers to grow on top of this 13 domain, rather than on the Au surface.41 This is consistent with the fact that nucleation has higher 14 15 energy barriers than growth processes, in such a way that nucleation and growth can be 16 17 separated.42 At intermediate conditions of pH, multiple nucleation points coexist at the Au NC 18 surface initially forming clover-like (discontinuous) shells that ultimately lead to the core@shell 19 20 when they grow further.41 The possibility to adjust the morphology of the hybrid structure and 21 22 obtain NCs with explicitly different coupling manners between the Au NCs and CeO2 allows 23 adjusting the properties of the NC. Thus, well-defined Au/CeO2 heterodimers, where parts of the 24 25 Au surface are exposed, experience smaller red-shifts than NCs with a uniform CeO2 coating 26 27 around the Au cores (Fig. S10). 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 Figure 4. Variation of the architecture of Au/CeO2 Hybrid NCs. Overall formation mechanism 51 (A) and representative SEM, TEM, and HAADF-STEM images (B, C) of Au/CeO2 hybrid NCs 52 obtained by adjusting the pH of the reaction solution. At acidic pH, Ce3+ is not oxidized and no 53 CeO2 deposits onto the Au NCs. At neutral and alkaline pH, Ce4+-citrate is formed. Its reactivity 54 depends on the pH of the solution. At neutral pH, Ce4+-citrate complexes slowly hydrolyze and 55 condensate onto Au NCs leading to heterostructured morphologies (B). At alkaline pH, the 56 57 hydrolysis and precipitation are faster and core@shell structures are promoted (C). At highly 58 alkaline pH, the complexing effect of citrate strongly decreases because of the high concentration 59 of competing hydroxides in solution. As a result, the homogeneous nucleation of isolated CeO2 60 NCs is not completely prevented. 11 ACS Paragon Plus Environment Chemistry of Materials Page 12 of 19 1 2 3 The versatility of the method allows the precise and deliberate manipulation of the NC 4 5 architecture by the simultaneous injection of Au3+ or Ce3+ precursors onto preformed clover-like 6 7 Au@CeO2 NCs. As expected, under appropriate conditions, Ce3+ oxidizes and deposits onto the 8 existent CeO2 domains. This process confines the overgrowth of Au onto the accessible ceria-free 9 10 surface of the hybrid NCs, thereby forcing the formation of highly anisotropic star-like NCs (Fig. 11 12 5). This change in morphology result into the emergence of a characteristic LSPR band in the 13 region between 600-800 nm (Fig. 5E).44 This intense broadband at the near-infrared region 14 15 together with the larger interface of anisotropic structures make these NCs promise candidates 16 17 for NIR-light-driven applications. 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 Figure 5. Branched Au/CeO2 NCs. Precise and deliberate manipulation of the NC architecture by 40 the simultaneous co-injection of Au3+ and Ce3+ precursors onto preformed Au/CeO2 NCs (A). 41 Representative TEM and HAADF_STEM images of clover-like Au@CeO2 NCs obtained by the 42 injection of Ce3+ precursor onto 15 nm Au NC seeds (B), and after 1st (C) and 2nd (D) co-injection. 43 The large lattice mismatch between CeO2 and Au favors the epitaxial reduction of Au3+ onto the 44 accessible Au domains of the structure and the formation of well-defined branched Au/CeO2 45 NCs. UV-vis spectra of as-obtained samples (E). The broadband in the region between 600-800 46 47 nm is assigned to the collective LSPR of the anisotropic Au domain. Additional morphological 48 characterization can be found in Fig. S12. 49 50 51 The presented approach can be extended to other systems. In preliminary experiments, similar 52 53 citrate-stabilized Ag/CeO2 NCs were obtained by replacing the Au seeds by Ag seeds (Fig. 6A). 54 55 Similarly, the atom-by-atom CeO2 deposition occurs and can be controlled also in the presence of 56 57 surfactants with strong or mild affinity for metal surfaces. Thus, 11-mercaptoundecanoic acid 58 (MUA)- and polyvinylpyrrolidone (PVP)-coated Au seeds were also successfully employed to 59 60 grow Au@CeO2 NCs (Fig. 6B-C). This point suggests that the method may be adapted to the CeO2 12 ACS Paragon Plus Environment Page 13 of 19 Chemistry of Materials 1 2 3 coating of anisotropic NC seeds, like cubes and rods, obtained by selective attachment of 4 5 surfactants to certain crystal facets, or the template-induced growth processes. 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 Figure 6. Representative TEM, SEM and HAADF-STEM images and corresponding UV-vis 30 spectra (A) of core@shell and heterodimer Ag/CeO2 NCs obtained using 30 nm Ag seeds. 31 Representative TEM images of Au@CeO2 NCs obtained using 11-mercaptoundecanoic- (B) and 32 33 10 KDa PVP- (C) stabilized 30 nm Au seeds. Synthesis conditions are detailed in the supporting 34 information. 35 36 37 38 39 With the aim of gaining further insight into the impact of the CeO2 coating on the optical 40 properties of Au NCs, extinction efficiencies of Au spheres of varying diameters and CeO2 coating 41 42 thickness have been calculated following the standard Mie theory45. Figure 7 plots individual 43 44 absorbance spectra, containing both absorption and scattering contributions, calculated for Au 45 spheres of different sizes (5, 10, 15, 30, 40 and 100 nm) coated with a homogeneous CeO2 shell of 46 47 different thicknesses (from 0.5 to 20 nm). Results are further summarized by plotting the position 48 49 of the LSPR band of Au NCs for the different CeO2 thicknesses (Fig. 7G). As can be seen, when 50 the Au cores are coated with the CeO2 dielectric shell of increasing thickness, the LSPR position 51 52 shifts gradually towards longer wavelengths and the absorbance increases, saturating at a 53 54 thickness when the electric field of the plasmon is fully confined inside the dielectric CeO2 layer. 55 Remarkably, both features (redshift and increase in peak intensity) depend on the size of the Au 56 57 NC core and can be explained in terms of the size-dependent near-field profiles of the Au NCs.46 58 59 Despite the good correlation between experimental results and Mie calculations, the calculated 60 13 ACS Paragon Plus Environment Chemistry of Materials Page 14 of 19 1 2 3 LSPR red-shits are systematically larger than those experimentally obtained. These differences 4 5 are mainly attributed to the “compactness” of the CeO2 shell. We certainly assume in our 6 7 calculations that Au NCs are surrounded by a homogeneous CeO2 coating layer while 8 experimental results suggest that the CeO2 shell is rather granular (porous) and of low density. 9 10 This assumption determines the effective dielectric constant of the system, which ultimately 11 12 affects the extent of the LSPR red-shifts (Fig. S14). Although the limitations of the Mie theory, 13 especially for small NC sizes, the good correlation between experimental results and Mie 14 15 calculations confirms the working hypothesis, that is, the possibility of precisely controlling the 16 17 strongly enhance local electromagnetic fields near the metal-dielectric interface by adjusting Au 18 core size, and CeO2 shell thicknesses. 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Figure 7. Influence of CeO2 shell thickness on the optical properties of Au NCs of different sizes. 55 Calculated absorbance spectra of Au spheres before and after their coating with a CeO2 shell of 56 57 different thickness (as-labelled). 5 nm (A), 10 nm (B), 15 nm (C), 30 nm (D), 40 nm (E), 100 nm (F). 58 Dependence of the surface plasmon resonance peaks positions for Au NCs of varying sizes (G). 59 Calculations were performed considering colloidal solutions of NCs (3x1012 NCs/mL) coated by 60 a CeO2 shell of refractive index 2.2 (20 °C) and dispersed in water (RI = 1.333, 20 °C).47 For citrate- 14 ACS Paragon Plus Environment Page 15 of 19 Chemistry of Materials 1 2 3 stabilized Au NCs, a shell thickness of 0.7 nm and RI=1.3735 was used (SI, section 15). Results 4 reveal the higher sensitivity of the smaller NCs in comparison with the larger sizes for a 5 determined CeO2 shell thickness. Similar calculations were performed to study the influence of 6 7 the shell coating on the LSPR peak of Au and Ag NCs of different sizes.31, 46 8 9 CONCLUSIONS 10 11 To summarize, we have developed a novel and simple seeded-growth aqueous approach for the 12 production of well-defined colloidal-stable citrate-stabilized Au/CeO2 hybrid nanocrystals (NCs) 13 14 with unprecedented control of their architectural and morphological characteristics. The 15 synthetic strategy, based on the identification of the experimental conditions under which the 16 heterogeneous nucleation and growth processes of CeO2 onto pre-synthetized Au NCs is 17 controlled, relies on the use of SC which plays multiple key roles as a reducer, pH buffer, 18 19 complexing, directing and stabilizing agent. The robustness and versatility of the synthetic 20 reaction allows for the fine adjustment of each individual domain in the structure, particularly, 21 the size of the Au core (from 5 nm to 100 nm), the thickness of the CeO2 shell (from 5 nm to 20 22 23 nm) and the growth mode of CeO2 onto Au (core@shell, clover-like, star-like or heterodimer). 24 Also, the approach allows for studying the impact of fine variations of hybrid NCs on the 25 underlying optical response. 26 27 Additionally, the level of morphological control obtained, as well as the ease by which such well- 28 29 defined nanostructures are produced, opens new opportunities for understanding the nanoscale 30 mechanisms and systematically investigate the interactions between individual components 31 allowing designing new catalysts and optical devices. Remarkably, since no organic solvents are 32 33 used and no toxic waste is formed during the reaction, the proposed synthesis method can be 34 defined as a sustainable, viable and low-cost strategy to fabricate noble metal/metal oxide NCs. 35 We believe that the results and understanding acquired in this study will contribute to the 36 development of a new generation of highly multifunctional complex hybrid NCs for a wide 37 38 variety of applications. 39 40 SUPPORTING INFORMATION 41 42 Details of experimental protocols and conditions studied together with extended NCs 43 44 characterization are available free of charge via the internet at http://pubs.acs.org. 45 46 ACKNOWLEDGMENTS 47 48 We acknowledge financial support from the Spanish MINECO (MAT2015-70725-R, ENE2017-85087- 49 50 C3), MICINN (RTI2018-099965-B-I00) and from the Catalan Agència de Gestió d'Ajuts Universitaris i 51 de Recerca (AGAUR) (2017-SGR-1431, 2017-SGR-327). Financial support from the HISENTS 52 (685817) Project financed by the European Community under H20202 Capacities Programme is 53 gratefully acknowledged. N.G.B. acknowledges financial support by MINECO through the 54 55 Ramon y Cajal program (RYC-2012- 10991). ICN2 is supported by the Severo Ochoa program 56 from Spanish MINECO (Grant No. SEV-2017-0706) and is funded by the CERCA Programme / 57 Generalitat de Catalunya. Part of the present work has been performed in the framework of the 58 59 Universitat Autònoma de Barcelona Degree and Ph.D. program. 60 15 ACS Paragon Plus Environment Chemistry of Materials Page 16 of 19 1 2 3 REFERENCES 4 5 1. Bastús, N. G.; Gonzalez, E.; Esteve, J.; Piella, J.; Patarroyo, J.; Merkoçi, F.; Puntes, V., 6 Exploring New Synthetic Strategies for the Production of Advanced Complex Inorganic 7 Nanocrystals. Z. Phys. Chem. 2015, 229 (1-2), 65-83. 8 9 2. Cozzoli, P. 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