Nanocluster growth via “graft-onto”: effects on geometric structures and optical properties

Abstract

Atomically precise engineering on the nanocluster surface remains highly desirable for the fundamental understanding of how surface structures of a nanocluster contribute to its overall properties. In this paper, the concept of “graft-onto” has been exploited to facilitate nanocluster growth on surface structures. Specifically, the Ag₂(DPPM)Cl₂ complex is used for re-constructing the surface structure of Pt₁Ag₂₈(SR)₁₈(PPh₃)₄ (Pt₁Ag₂₈, SR = 1-adamantanethiolate) and producing a size-growth nanocluster – Pt₁Ag₃₁(SR)₁₆(DPPM)₃Cl₃ (Pt₁Ag₃₁). The grafting effect of Ag₂(DPPM)Cl₂ induces both direct changes on the surface structure (e.g., size growth, structural transformation, and surface rotation) and indirect changes on the kernel structure (from a fcc configuration to an icosahedral configuration). Remarkable differences have been observed by comparing optical properties between Pt₁Ag₂₈ and Pt₁Ag₃₁. Significantly, Pt₁Ag₃₁ exhibits high photo-luminescent intensity with a quantum yield of 29.3%, which is six times that of the Pt₁Ag₂₈. Overall, this work presents a new approach (i.e., graft-onto) for the precise dictation of nanocluster surface structures at the atomic level.

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1 Introduction

It has long been nano-scientists' dream to control compositions and structures of nanomaterials at the atomic level. Through the continuous accumulation of synthetic experience and with the help of advanced analytical methods, researchers can now easily tailor the composition and the morphology of metal nanoparticles.^1,2 However, it still remains extremely difficult to realize the atomic-level tailoring of specific sites on the nanoparticle surface (for example, adding or deleting one or two metal atoms at a designated position), which is the most relevant to the physical–chemical properties of such nanomaterials.

Nanoclusters (so-called ultrasmall nanoparticles) are an emerging class of promising nanomaterials owing to their atomically precise structures and intriguing properties.^3,4 Because of the quantum size effect of nanoclusters, any perturbations on compositions/structures may induce tremendous variations on clusters' properties.^3–6 Knowledge and understanding of these structure–property correlations are keys to the ultimate goal in the nanocluster science – arbitrarily dictating the properties via precisely tailoring the structures. Exploring effective approaches to exquisitely tailor the size, structure, and composition of atomically precise nanoclusters is a prerequisite to achieve this goal.

In recent years, ligand engineering has served as an efficient approach to convert structures of nanoclusters.⁷ In general, great structural transformation occurs when the peripheral ligands of nanoclusters are drastically substituted by the introduced ligands (for instance, from mono-icosahedral Au₁₁(PPh₃)₃Cl₃ to bi-icosahedral Au₂₅(PPh₃)₁₀(SR)₅Cl₂,^7a from Ag₄₄(SR)₃₀ with a hollow kernel to Ag₂₅(SR)₁₈ with a nonhollow kernel,^7b and so on). However, it still remains challenging to tailor specific sites on the nanocluster surface without constructing the overall structure. A new approach for tailoring the nanocluster surface is highly desirable for the fundamental understanding of how surface structures in a nanocluster contribute to its overall properties.

By noting two facts about nanoclusters that (i) it is hard to substitute thiol ligands on the nanocluster surface by the introduced phosphorus ligands (because the metal–S covalent bond is more robust than the metal–P coordination bond) and (ii) several thiolated nanoclusters are terminally capped by metal–PPh₃ units (such as Pt₁Ag₂₈(SR)₁₈(PPh₃)₄, Ag₂₉(SSR)₁₂(PPh₃)₄, and Ag₃₃(SR)₂₄(PPh₃)₄ clusters with Ag–PPh₃ terminals),⁸ we perceive a good opportunity to re-construct the nanocluster surface without largely affecting its whole structure – substituting these terminal metal–PPh₃ units by bidentate phosphorus metal complexes. Such a substitution may not only fine-tune the nanocluster surface structure, but also shed light on structure–property correlations at the atomic level.

In the current work, we report a “graft-onto” strategy to facilitate a controllable size-growth of the nanocluster surface. Induced by the addition of the Ag₂(DPPM)Cl₂ complex (DPPM = bis-(diphenylphosphino)-methane), Pt₁Ag₂₈(SR)₁₈(PPh₃)₄ (Pt₁Ag₂₈; SR = 1-adamantanethiolate) converts into a size-growth nanocluster – Pt₁Ag₃₁(SR)₁₆(DPPM)₃Cl₃ (Pt₁Ag₃₁). Great changes (including size growth, structural transformation, and surface rotation) take place on the outermost shell of Pt₁Ag₂₈, owing to the direct grafting effect of the Ag₂(DPPM)Cl₂. The changes on the outermost shell further induce the transformation of the innermost Pt₁Ag₁₂ kernel from a fcc configuration in Pt₁Ag₂₈ to an icosahedral configuration in Pt₁Ag₃₁. Pt₁Ag₂₈ and Pt₁Ag₃₁ nanoclusters reflect remarkable differences in both optical absorption and PL emission. Significantly, Pt₁Ag₃₁ displays high photo-luminescence (PL) intensity with a quantum yield (QY) of 29.3%, which is six times that of the Pt₁Ag₂₈ (PL QY = 4.9%).

2 Experimental methods

Materials

All reagents were purchased from Acros Organics and Sigma-Aldrich and used without further purification: hexachloroplatinic(IV) acid (H₂PtCl₆·6H₂O, 99%, metals basis), silver nitrate (AgNO₃, 99% metals basis), adamantane-1-thiol (C₁₀H₁₅SH, HS-Adm, 95%), triphenylphosphine (PPh₃, 95%), bis(diphenylphosphino)methane ((C₆H₅)₂PCH₂P(C₆H₅)₂, DPPM, 98%), sodium borohydride (NaBH₄, 99.9%), sodium chloride (NaCl, 99.5%), sodium hexafluoroantimonate (NaSbF₆, 99%), rhodamine B (RB, for fluorescence), methylene chloride (CH₂Cl₂, HPLC, Aldrich), methanol (CH₃OH, HPLC, Aldrich), ethyl acetate (CH₃COOC₂H₅, HPLC, Aldrich), ethanol (CH₃CH₂OH, HPLC, Aldrich), ether (C₂H₅OC₂H₅, HPLC, Aldrich), and 2-methyltetrahydrofuran (C₄H₇O-2-CH₃, HPLC, Aldrich).

Synthesis of the [Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄]Cl₂ nanocluster

For the nanocluster synthesis, AgNO₃ (29 mg, 0.17 mmol) and H₂PtCl₆·6H₂O (5 mg, 0.01 mmol) were dissolved in CH₃OH (5 mL) and CH₃COOC₂H₅ (35 mL). The solution was vigorously stirred (1200 rpm) with magnetic stirring for 15 min. Then, Adm-SH (0.1 g) and PPh₃ (0.1 g) were added and the reaction was vigorously stirred (1200 rpm) for another 90 min. After this, NaBH₄ aqueous solution (1 mL, 20 mg mL⁻¹) was added quickly to the above mixture. The reaction was allowed to proceed for 36 h under a N₂ atmosphere. After this, the aqueous layer was removed, and the mixture in the organic phase was rotavaporated under vacuum. Then approximately 30 × 3 mL of CH₃CH₂OH was used to wash the obtained nanoclusters. The precipitate was dissolved in CH₂Cl₂, which produced the [Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄]Cl₂ nanocluster. The yield is 45% based on the Ag element (calculated from AgNO₃) for the synthesis of [Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄]Cl₂.

Synthesis of the Ag₂(DPPM)Cl₂ complex

0.17 g of AgNO₃ (1 mmol) was dissolved in 50 mL of CH₃CH₂OH, and NaCl aqueous solution (6 mL, 10 mg mL⁻¹) was added quickly to the above mixture. The reaction was stirred for 1 minute. The white precipitate was collected and rotavaporated under vacuum, which produced the AgCl powder. Then, 0.07 g of AgCl (0.5 mmol) was dispersed in 20 mL of CH₂Cl₂, to which solution 0.19 g DPPM was added. The reaction was vigorously stirred (1200 rpm) with magnetic stirring for 30 minutes. After this, the solution was evaporated to dryness, which produced the Ag₂(DPPM)Cl₂ complex. The yield is about 95% based on the Ag element (calculated from AgCl) for the synthesis of Ag₂(DPPM)Cl₂.

Synthesis of the [Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃]Cl₄ nanocluster

For the nanocluster synthesis, 30 mg of [Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄]Cl₂ was dissolved in 30 mL of CH₂Cl₂, to which 10 mg of Ag₂(DPPM)Cl₂ was added. The reaction was allowed to proceed for 10 minutes at room temperature. After this, the organic layer was separated from the precipitate and evaporated to dryness. Then, approximately 30 × 3 mL of CH₃CH₂OH was used to wash the obtained nanoclusters. The precipitate was dissolved in CH₂Cl₂, which produced the [Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃]Cl₄ nanocluster. The yield is about 85% based on the Ag element (calculated from the Pt₁Ag₂₈) for the synthesis of [Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃]Cl₄.

Single-crystal growth of [Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃](SbF₆)₄

For accelerating the crystallization process and improving the quality of crystals, the counterion Cl⁻ in [Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃]Cl₄ was replaced by SbF₆⁻. The reaction equation is [Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃]Cl₄ + 4 SbF₆⁻ → [Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃](SbF₆)₄ + 4Cl⁻. Specifically, 20 mg of [Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃]Cl₄ was dissolved in 20 mL of CH₂Cl₂. Then, 1 mL of NaSbF₆–CH₃CH₂OH solution (5 mg mL⁻¹) was added. After 3 minutes, the organic layer was separated from the precipitate and evaporated to dryness. The precipitate was dissolved in CH₂Cl₂, which produced the [Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃](SbF₆)₄ nanocluster. Nanoclusters were crystallized in a CH₂Cl₂/ether system with a vapor diffusion method. Specifically, 20 mg of clusters was dissolved in 5 mL of CH₂Cl₂, and the obtained solution was then vapor diffused using 50 mL of ether. After 3 days, dark red crystals of Pt₁Ag₃₁ were collected and subjected to X-ray diffraction to determine the structure. The CCDC number of [Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃](SbF₆)₄ is 1937755. Notably, the optical absorption and PL emission properties of the Pt₁Ag₃₁ nanocluster remain unchanged after the counter-ion replacement.

Test of the temperature-photoluminescence (PL) intensity correlation

The nanocluster (0.1 mg) was dissolved in 5 mL of the CH₂Cl₂/C₄H₇O-2-CH₃ (v/v = 1 : 1) mixture. Then, the solutions were cooled from 293 K to different temperatures and the PL spectra were measured.

X-ray crystallography

The data collection for single crystal X-ray diffraction was carried out on a Bruker Smart APEX II CCD diffractometer under a nitrogen flow at 170 K, using graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å). Data reductions and absorption corrections were performed using the SAINT and SADABS programs, respectively.^9a The electron density was squeezed using PLATON, and detailed information can be found in Table S3.† The structure was solved by direct methods and refined with full-matrix least squares on F² using the SHELXTL software package.^9b All non-hydrogen atoms were refined anisotropically, and all the hydrogen atoms were set in geometrically calculated positions and refined isotropically using a riding model.

Theoretical methods

Density functional theory (DFT) calculations were employed to optimize the geometric structures and calculated the Kohn–Sham orbitals of Pt₁Ag₂₈ and Pt₁Ag₃₁ nanoclusters using the Perdew–Burke–Ernzerhof (PBE) GGA functional.^10a The triple-zeta polarized (TZP) basis set with inclusion of the scalar relativistic effect via a zeroth-order regular approximation (ZORA) implemented in the ADF package was adopted.^10b

Characterization

All UV-vis absorption spectra of the nanoclusters dissolved in CH₂Cl₂ were recorded using an Agilent 8453 diode array spectrometer, whose background correction was made using a CH₂Cl₂ blank.

Photo-luminescence (PL) spectra were measured on an FL-4500 spectrofluorometer with the same optical density (OD) of 0.05. In these experiments, the nanocluster solutions were prepared in CH₂Cl₂ at a concentration of less than 1 mg mL⁻¹.

Absolute quantum yield (QY) was measured with dilute solutions of nanoclusters on a HORIBA FluoroMax-4P. For determining the QYs of clusters, the nanocluster solutions were prepared in CH₂Cl₂ with the same OD of 0.05. Besides, the PL comparison between the Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃ nanocluster and rhodamine B was performed, to further determine the PL QY of the Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃ nanocluster.

Thermogravimetric analysis (TGA) was carried out on a thermogravimetric analyzer (DTG-60H, Shimadzu Instruments, Inc.). 10 mg of clusters was used for collecting the TGA data on clusters.

X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 configured with a monochromated Al Kα (1486.8 eV) 150 W X-ray source, 0.5 mm circular spot size, a flood gun to counter charging effects, and analysis chamber base pressure lower than 1 × 10⁻⁹ mbar.

Inductively coupled plasma-atomic emission spectrometry (ICP-AES) measurements were performed on an Atomscan advantage instrument from Thormo Jarrell Ash Corporation (USA).

Elemental analysis (EA) was performed on Vario EL cube. 3 mg of each cluster sample was used for collecting the EA data.

Energy-dispersive X-ray spectroscopy (EDS) analyses were performed on a JEOL JEM-2100F FEG TEM operated at 200 kV. Nanocluster powder samples were used for the analysis.

Electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) measurements were performed using a MicrOTOF-QIII high-resolution mass spectrometer; for preparing the ESI sample, the clusters were dissolved in CH₂Cl₂ (1 mg mL⁻¹) and diluted (v/v = 1 : 2) with methanol.

3 Results and discussion

Pt₁Ag₂₈ was prepared first via an in situ synthetic procedure (see the Experimental section for more details). A combination of ESI-MS, UV-vis absorption, PL, XPS, ICP, and EA results unambiguously identified that the obtained Pt₁Ag₂₈ nanocluster is the same as the one reported previously (Fig. S1, and Tables S1, S2†).^8h The EDS results demonstrated the presence of Cl in the cluster system, which was considered as the counterion for the Pt₁Ag₂₈ nanocluster, namely, [Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄]Cl₂ (Fig. S2†).

The reaction between Pt₁Ag₂₈ and Ag₂(DPPM)Cl₂ generates a Pt₁Ag₃₁ nanocluster, wherein the Ag–PPh₃ vertexes were substituted by the Ag₂(DPPM)Cl, accompanied by a size-growth of the metallic kernel from M₂₉ to M₃₂ (M = Pt/Ag; Fig. 1). As for the overall structure, the Ag–PPh₃ terminals (in Pt₁Ag₂₈) are bonded onto the nanoclusters with an I-type growth mode, whereas the Ag₂(DPPM)Cl terminals (in Pt₁Ag₃₁) follow a Y-type growth mode (Fig. 1). The transformation of the nanocluster terminal from the single-linked Ag–PPh₃ (I type; see Fig. 1A) into the double-linked Ag₂(DPPM)Cl (Y type; see Fig. 1B) reflects the “graft-onto growth” and accounts for the size growth, structural transformation, and surface rotation of the nanocluster.

Fig. 1 Transformation from Pt₁Ag₂₈ into Pt₁Ag₃₁. (A) Schematic illustration of the I-type growth of the Ag–PPh₃ terminal and the structure of Pt₁Ag₂₈. (B) Schematic illustration of the Y-type growth of the Ag₂(DPPM)Cl terminal and the structure of Pt₁Ag₃₁. In this transformation process, the Ag–PPh₃ terminals in Pt₁Ag₂₈ are peeled off, and the Ag₂(DPPM)Cl terminals are introduced. Color legends: dark green sphere, Pt; blue sphere, Ag; red sphere, S; purple sphere, P; green sphere, Cl; grey sphere, C. For clarity, all H atoms and some C atoms are omitted.

The ESI-MS and EDS results identified the molecular formula as [Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃]Cl₄ (Fig. S3 and S4†). Both Pt₁Ag₂₈ and Pt₁Ag₃₁ clusters contain 8 free valence electrons – (i) for Pt₁Ag₂₈, the free electron count is 28(Ag) − 18(SR) − 2(charge) = 8; (ii) for Pt₁Ag₃₁, the free electron count is 31(Ag) − 16(SR) − 3 (Cl) − 4(charge) = 8. The atomic ratio of Pt to Ag in Pt₁Ag₃₁ was analyzed by XPS and ICP, and the experimental results were consistent with the theoretical ratio (Fig. S5, and Table S1†). TGA of the Pt₁Ag₃₁ nanocluster showed a total weight loss of 52.65%, matching the theoretical value of 53.54% (the proportion of SR, DPPM, Cl ligands and the Cl counterion in the overall formula; Fig. S6†).

The structural comparison between Pt₁Ag₂₈ and Pt₁Ag₃₁ is shown in Fig. 2 (see Fig. S7† for the total structure of Pt₁Ag₃₁). Pt₁Ag₂₈ comprises a fcc Pt₁Ag₁₂ kernel, a trilateral Ag₁₂(SR)₁₅(PPh₃)₃ shell, and a helical Ag₄(SR)₃(PPh₃)₁ unit (Fig. 2A–F). The trilateral Ag₁₂(SR)₁₅(PPh₃)₃ shell is constituted by assembling of three same Ag₄(SR)₆(PPh₃)₁ units (Ag₃(SR)₆ face + Ag–PPh₃ terminals) by sharing the terminal thiol ligands (Fig. 2B and M).

Fig. 2 Structural anatomies of Pt₁Ag₂₈ and Pt₁Ag₃₁ nanoclusters. (A–F and M) Structural anatomy of the Pt₁Ag₂₈ nanocluster. (G–L and N) Structural anatomy of the Pt₁Ag₃₁ nanocluster. Color legends: dark green sphere, Pt; blue/dark grey/orange/dark blue sphere, Ag; brown sphere, motif-shared Ag; red sphere, S; pink sphere, motif-shared S; purple sphere, P; green sphere, Cl; grey sphere, C. For clarity, all H atoms and some C atoms are omitted.

With the grafting effect, the vertex Ag-PPh₃ units on the Pt₁Ag₂₈ surface are peeled off, and Ag₂(DPPM)Cl units are introduced. As a result, three additional Ag atoms (Pt₁Ag₃₁ − Pt₁Ag₂₈ = 3 Ag) are incorporated onto the nanocluster surface because of the grafting effect of bidentate DPPM ligands. Three Cl ligands are also introduced to further stabilize the surface structure of Pt₁Ag₃₁. More specifically—

(i) Although the composition of the Pt₁Ag₁₂ kernel maintains throughout the graft-onto process, the fcc configuration of the Pt₁Ag₁₂ kernel in Pt₁Ag₂₈ alters to an icosahedral configuration in Pt₁Ag₃₁ (Fig. 2A and G). The average bond length between Pt(core) and Ag(kernel surface) in Pt₁Ag₃₁ is smaller than that in Pt₁Ag₂₈, whereas the bonds between Ag(kernel surface) and Ag(kernel surface) in Pt₁Ag₃₁ are much longer than those in Pt₁Ag₂₈ (Table 1 and Fig. S8†).

Table 1 Comparison of bond lengths in Pt₁Ag₂₈ and Pt₁Ag₃₁ nanoclusters. Such bonds are highlighted in Fig. S8

	Pt1Ag28	Pt1Ag31	Diff.
Pt(core)–Ag(kernel surface) bond	2.768–2.797 Å (Avg. 2.838 Å)	2.735–2.786 Å (Avg. 2.760 Å)	−2.83%
Ag(kernel surface)–Ag (kernel surface) bond	2.751–2.848 Å (Avg. 2.802 Å)	2.817–3.144 Å (Avg. 2.906 Å)	+3.58%
Ag(kernel surface)–S(motif) bond	2.438–2.498 Å (Avg. 2.472 Å)	2.445–2.591 Å (Avg. 2.495 Å)	+0.92%
Ag(motif)–S(motif) bond	2.254–2.992 Å (Avg. 2.560 Å)	2.356–2.835 Å (Avg. 2.460 Å)	+4.07%
Ag(motif)–P(vertex) bond	2.292–2.384 Å (Avg. 2.356 Å)	2.397–2.428 Å (Avg. 2.405 Å)	+2.04%

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(ii) The trilateral Ag₁₅(SR)₁₃(DPPM)₃Cl₃ shell in Pt₁Ag₃₁ is constituted by the assembly of three same Ag₆(SR)₆(DPPM)₁Cl₁ units via sharing Ag₂(SR)₃ edges (Fig. 2H and N). Due to the steric hindrance effect, only 13 thiol ligands exist in the trilateral Ag₁₅(SR)₁₃(DPPM)₃Cl₃ shell and less than the 15 thiol ligands in the Ag₁₂(SR)₁₅(PPh₃)₃ trilateral shell of Pt₁Ag₂₈ (Fig. 2B and H). In each Ag₆(SR)₆(DPPM)₁Cl₁ unit, the Cl ligand fixes two Ag atoms that bond with the DPPM ligand (Fig. 2N). All Ag-ligand interactions (including Ag(kernel surface)–S(motif), Ag(motif)–S(motif), and Ag(motif)–P(vertex)) in Pt₁Ag₃₁ are longer than those in Pt₁Ag₂₈ (Table 1).

(iii) The Pt₁Ag₂₄(SR)₁₅(PPh₃)₃ structure in Pt₁Ag₂₈ is covered by a helical Ag₄(SR)₃(PPh₃)₁ unit (Fig. 2D), whereas the corresponding structure in Pt₁Ag₃₁ is just Ag₄(SR)₃ (Fig. 2J); that is, the terminal PPh₃ ligand is peeled off. A similar situation has recently been observed in the transformation of Ag₂₉(SSR)₁₂(PPh₃)₄ into Cs₃Ag₂₉(SSR)₁₂(DMF)_x.¹¹ For the Cs₃Ag₂₉(SSR)₁₂(DMF)_x nanocluster, because of the absence of the vertex PPh₃ ligand, the terminal Ag atom becomes closer to the innermost Ag₁₃ kernel.¹¹ A similar situation has been observed in this work – the average distance between the terminal Ag and adjacent Ag atoms in the innermost Pt₁Ag₁₂ kernel in Pt₁Ag₃₁ is 4.026 Å, which is much shorter than that in Pt₁Ag₂₈ (4.290 Å, as shown in Fig. S9†). In this context, the terminal Ag-based structure in Pt₁Ag₃₁ becomes more contractive for reducing the exposure of this bare Ag atom, which in turn makes the overall structure more robust.

Collectively, the “graft-onto” process on the Pt₁Ag₂₈ surface changes the vertex structure from PPh₃-Ag to DPPM-Ag₂-Cl, resulting in the size-growth and surface structural transformation of the nanocluster. The transformation of the outermost shell further induces changes on kernel–shell interactions, and such changes alter the innermost Pt₁Ag₁₂ kernel from a fcc configuration in Pt₁Ag₂₈ to an icosahedral configuration in Pt₁Ag₃₁.

From the structural point of view, aside from Pt₁Ag₂₈(SR)₁₈(PPh₃)₄, several other metal nanoclusters are terminally capped by metal–PPh₃, such as Ag₂₉(SSR)₁₂(PPh₃)₄, Ag₃₃(SR)₂₄(PPh₃)₄, Au₂₃(PhC≡C)₉(PPh₃)₆, Au₂₄(PhC≡C)₁₄(PR)₄, and so on.⁸ Our reported “graft-onto” strategy might also be applicable in these cluster systems for controlling their surface structures. Future work will focus on extending the “graft-onto” strategy to other cluster systems.

Both Pt₁Ag₂₈ and Pt₁Ag₃₁ nanoclusters are stable in DMF at 50 °C for at least 24 hours (Fig. S10A and C†). At 80 °C, the optical absorptions of Pt₁Ag₂₈ disappear over time (Fig. S10B†); by comparison, the Pt₁Ag₃₁ nanocluster is stable enough to maintain its optical absorptions (Fig. S10D†). We propose that the enhanced thermal stability of Pt₁Ag₃₁ results from its more robust structure – compared with PPh₃, the introduced DPPM ligands have more ability to fix the nanocluster surface and thus suppress the vibration of the overall structure.

The optical properties of Pt₁Ag₂₈ and Pt₁Ag₃₁ nanoclusters are compared. Optical absorption of Pt₁Ag₂₈ shows an intense peak at 445 nm and a shoulder peak at 540 nm. The transformation of Pt₁Ag₂₈ into Pt₁Ag₃₁ results in an obvious blue-shift for each peak – the peak at 445 nm blue-shifts to 430 nm and becomes wider, and the shoulder band at 540 nm blue-shifts to 525 nm (Fig. 3A). The blue shift of the maximum optical absorption of nanoclusters (i.e., from 540 nm of Pt₁Ag₂₈ to 525 nm of Pt₁Ag₃₁) always represents the enlargement of the HOMO–LUMO energy gap (HOMO: the highest occupied molecular orbital; LUMO: the lowest unoccupied molecular orbital), which matches the DFT calculation results that Pt₁Ag₃₁ displays a larger energy gap relative to Pt₁Ag₂₈ (1.92 eV versus 1.76 eV, Fig. S11†).

Fig. 3 Optical properties of nanoclusters. (A) Optical absorptions of Pt₁Ag₂₈ and Pt₁Ag₃₁ nanoclusters. (B) PL emissions of Pt₁Ag₂₈ and Pt₁Ag₃₁ nanoclusters. Insets in B: peak shift in normalized PL spectra, and digital photo of each cluster in CH₂Cl₂ under UV light.

The Pt₁Ag₂₈ nanocluster (in CH₂Cl₂ solution) emits at 672 nm, with a photo-luminescence quantum yield (PL QY) of 4.9%.^8h By comparison, the Pt₁Ag₃₁ nanocluster (in CH₂Cl₂ solution, with the same OD as Pt₁Ag₂₈ solution) emits at 651 nm, representing a 21 nm blue shift relative to that of the Pt₁Ag₂₈. Significantly, the PL QY of Pt₁Ag₃₁ in CH₂Cl₂ is 29.3%; that is, the PL intensity of Pt₁Ag₃₁ is six times that of the Pt₁Ag₂₈. Such an enhancement can also be inferred from the PL spectra of two nanoclusters (Fig. 3B). Besides, the PL comparison between Pt₁Ag₃₁ and rhodamine B further determined the PL QY of the nanocluster (Fig. S12 and S13†). Under weak UV light, the emission of Pt₁Ag₂₈ is hard to be observed, whereas the PL of Pt₁Ag₃₁ is strong enough to be perceived with the naked eye (Fig. 3B, insets). Such a PL enhancement may result from the enhanced stabilization ability of DPPM relative to PPh₃ – the introduced DPPM ligand firmly fixes the surface structure and suppresses the vibration of the overall structure; in this context, the energy dissipation of the photo-excited Pt₁Ag₃₁ reduces with non-radiative transitions (mainly affected by intramolecular vibrations), but enhances with radiative transitions (through luminescence).

Temperature-dependent fluorescence of the Pt₁Ag₃₁ nanocluster was monitored. For the Pt₁Ag₂₈ nanocluster, our previous work demonstrated a 20-fold enhancement on the PL intensity of Pt₁Ag₂₈ (in DMF solution) along with the temperature-lowering process from 293 K to 125 K, and the PL QY increased from 9.3% to ∼100% (Fig. S14†).^12a As to the Pt₁Ag₃₁ nanocluster (Fig. 4), the PL intensity presented a 4.5-fold enhancement by comparing the 179 K data with the 293 K data (Fig. 4A–C), and the optical absorption just exhibited a 1.3-fold enhancement in the corresponding temperature-lowering process (Fig. 4D). Accordingly, the PL QY of Pt₁Ag₃₁ was almost 100% when the temperature was lower than 179 K. Such an enhancement of PL intensity resulted from the reduced energy consumption of thermal vibrations of nanoclusters (non-radiative transition) reduced by the reduced temperature; in this context, the excitation energy could only be released by the PL approach (radiative transition).^11,12

Fig. 4 Temperature-dependent PL of Pt₁Ag₃₁ (dissolved in CH₂Cl₂). (A) Temperature-dependent emission of Pt₁Ag₃₁. (B) The PL intensity of Pt₁Ag₃₁ at the fixed point of 651 nm. (C) The derivative results for the temperature-dependent PL intensity of Pt₁Ag₃₁. (D) Optical absorption of Pt₁Ag₃₁ at 293 K and 179 K.

4 Conclusions

In summary, a “graft-onto” strategy is presented for facilitating a controllable size-growth of the nanocluster surface. The addition of the Ag₂(DPPM)Cl₂ complex converts Pt₁Ag₂₈(S-Adm)₁₈(PPh₃)₄ into a size-growth nanocluster, namely, Pt₁Ag₃₁(S-Adm)₁₆(DPPM)₃Cl₃. Induced by the grafting effect, direct changes on the surface structure (e.g., size growth, structural transformation, and surface rotation) and indirect changes on the kernel structure (from a fcc configuration to an icosahedral configuration) take place. Obvious differences have been observed by comparing the optical properties (optical absorption and PL emission) of two nanoclusters. Significantly, Pt₁Ag₃₁(SR)₁₆(DPPM)₃Cl₃ displayed a high PL intensity with a PL QY of 29.3%, which is six times that of the Pt₁Ag₂₈(SR)₁₈(PPh₃)₄. Our work presents a new strategy for controllably re-constructing the nanocluster surface at the atomic level, which hopefully sheds light on the fundamental understanding of how surface structures in a nanocluster contribute to its overall properties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support by the NSFC (U1532141, 21631001, 21871001, and 21803001), the Ministry of Education, the Education Department of Anhui Province (KJ2017A010), and 211 Project of Anhui University.

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Fig. S1–S14 and Tables S1–S3 for the EDS, ESI-MS, XPS, ICP, TGA, and stability and PL results of nanoclusters, and the structural comparison between nanoclusters. CCDC 1937755. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9sc05700e

‡ These authors contributed equally to this work.

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