Nanocluster growth via "graft-onto": effects on geometric structures and optical properties.

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. This journal is © The Royal Society of Chemistry 2020.

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)5Cl2,7 from Ag44(SR)30 with a hollow kernel to Ag25(SR)18 with a nonhollow kernel,7 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 Pt(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), Pt(SR)18(PPh3)4 (Pt; SR = 1-adamantanethiolate) converts into a size-growth nanocluster – Pt(SR)16(DPPM)3Cl3 (Pt). Great changes (including size growth, structural transformation, and surface rotation) take place on the outermost shell of Pt, 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 Pt to an icosahedral configuration in Pt. Pt and Pt nanoclusters reflect remarkable differences in both optical absorption and PL emission. Significantly, Pt displays high photo-luminescence (PL) intensity with a quantum yield (QY) of 29.3%, which is six times that of the Pt (PL QY = 4.9%).

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 [Pt(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 [Pt(S-Adm)18(PPh3)4]Cl2 nanocluster. The yield is 45% based on the Ag element (calculated from AgNO3) for the synthesis of [Pt(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 [Pt(S-Adm)16(DPPM)3Cl3]Cl4 nanocluster

For the nanocluster synthesis, 30 mg of [Pt(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 [Pt(S-Adm)16(DPPM)3Cl3]Cl4 nanocluster. The yield is about 85% based on the Ag element (calculated from the Pt) for the synthesis of [Pt(S-Adm)16(DPPM)3Cl3]Cl4.

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

For accelerating the crystallization process and improving the quality of crystals, the counterion Cl– in [Pt(S-Adm)16(DPPM)3Cl3]Cl4 was replaced by SbF6–. The reaction equation is [Pt(S-Adm)16(DPPM)3Cl3]Cl4 + 4 SbF6– → [Pt(S-Adm)16(DPPM)3Cl3](SbF6)4 + 4Cl–. Specifically, 20 mg of [Pt(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 [Pt(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 Pt were collected and subjected to X-ray diffraction to determine the structure. The CCDC number of [Pt(S-Adm)16(DPPM)3Cl3](SbF6)4 is ; 1937755. Notably, the optical absorption and PL emission properties of the Pt 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.9 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.9 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 Pt and Pt nanoclusters using the Perdew–Burke–Ernzerhof (PBE) GGA functional.10 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.10

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 Pt(S-Adm)16(DPPM)3Cl3 nanocluster and rhodamine B was performed, to further determine the PL QY of the Pt(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.

Results and discussion

Pt 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 Pt nanocluster is the same as the one reported previously (Fig. S1, and Tables S1, S2†).8 The EDS results demonstrated the presence of Cl in the cluster system, which was considered as the counterion for the Pt nanocluster, namely, [Pt(S-Adm)18(PPh3)4]Cl2 (Fig. S2†).

The reaction between Pt and Ag2(DPPM)Cl2 generates a Pt 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 Pt) are bonded onto the nanoclusters with an I-type growth mode, whereas the Ag2(DPPM)Cl terminals (in Pt) 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 Pt into Pt. (A) Schematic illustration of the I-type growth of the Ag–PPh3 terminal and the structure of Pt. (B) Schematic illustration of the Y-type growth of the Ag2(DPPM)Cl terminal and the structure of Pt. In this transformation process, the Ag–PPh3 terminals in Pt 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 [Pt(S-Adm)16(DPPM)3Cl3]Cl4 (Fig. S3 and S4†). Both Pt and Pt clusters contain 8 free valence electrons – (i) for Pt, the free electron count is 28(Ag) – 18(SR) – 2(charge) = 8; (ii) for Pt, the free electron count is 31(Ag) – 16(SR) – 3 (Cl) – 4(charge) = 8. The atomic ratio of Pt to Ag in Pt was analyzed by XPS and ICP, and the experimental results were consistent with the theoretical ratio (Fig. S5, and Table S1†). TGA of the Pt 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 Pt and Pt is shown in Fig. 2 (see Fig. S7† for the total structure of Pt). Pt 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 Pt and Pt nanoclusters. (A–F and M) Structural anatomy of the Pt nanocluster. (G–L and N) Structural anatomy of the Pt 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 Pt surface are peeled off, and Ag2(DPPM)Cl units are introduced. As a result, three additional Ag atoms (Pt – Pt = 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 Pt. More specifically—

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

Table 1

Comparison of bond lengths in Pt and Pt 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%

(ii) The trilateral Ag15(SR)13(DPPM)3Cl3 shell in Pt 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 Pt (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 Pt are longer than those in Pt (Table 1).

(iii) The Pt1Ag24(SR)15(PPh3)3 structure in Pt is covered by a helical Ag4(SR)3(PPh3)1 unit (Fig. 2D), whereas the corresponding structure in Pt 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).11 For the Cs3Ag29(SSR)12(DMF) 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 Pt is 4.026 Å, which is much shorter than that in Pt (4.290 Å, as shown in Fig. S9†). In this context, the terminal Ag-based structure in Pt 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 Pt 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 Pt to an icosahedral configuration in Pt.

From the structural point of view, aside from Pt(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(PhCC)9(PPh3)6, Au24(PhCC)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 Pt and Pt nanoclusters are stable in DMF at 50 °C for at least 24 hours (Fig. S10A and C†). At 80 °C, the optical absorptions of Pt disappear over time (Fig. S10B†); by comparison, the Pt nanocluster is stable enough to maintain its optical absorptions (Fig. S10D†). We propose that the enhanced thermal stability of Pt 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 Pt and Pt nanoclusters are compared. Optical absorption of Pt shows an intense peak at 445 nm and a shoulder peak at 540 nm. The transformation of Pt into Pt 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 Pt to 525 nm of Pt) 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 Pt displays a larger energy gap relative to Pt (1.92 eV versus 1.76 eV, Fig. S11†).

Fig. 3

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

The Pt nanocluster (in CH2Cl2 solution) emits at 672 nm, with a photo-luminescence quantum yield (PL QY) of 4.9%.8 By comparison, the Pt nanocluster (in CH2Cl2 solution, with the same OD as Pt solution) emits at 651 nm, representing a 21 nm blue shift relative to that of the Pt. Significantly, the PL QY of Pt in CH2Cl2 is 29.3%; that is, the PL intensity of Pt is six times that of the Pt. Such an enhancement can also be inferred from the PL spectra of two nanoclusters (Fig. 3B). Besides, the PL comparison between Pt and rhodamine B further determined the PL QY of the nanocluster (Fig. S12 and S13†). Under weak UV light, the emission of Pt is hard to be observed, whereas the PL of Pt 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 Pt reduces with non-radiative transitions (mainly affected by intramolecular vibrations), but enhances with radiative transitions (through luminescence).

Temperature-dependent fluorescence of the Pt nanocluster was monitored. For the Pt nanocluster, our previous work demonstrated a 20-fold enhancement on the PL intensity of Pt (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†).12 As to the Pt 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 Pt 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 Pt (dissolved in CH2Cl2). (A) Temperature-dependent emission of Pt. (B) The PL intensity of Pt at the fixed point of 651 nm. (C) The derivative results for the temperature-dependent PL intensity of Pt. (D) Optical absorption of Pt at 293 K and 179 K.

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 Pt(S-Adm)18(PPh3)4 into a size-growth nanocluster, namely, Pt(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, Pt(SR)16(DPPM)3Cl3 displayed a high PL intensity with a PL QY of 29.3%, which is six times that of the Pt(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.

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