The tetrahedral structure and luminescence properties of Bi-metallic Pt1Ag28(SR)18(PPh3)4 nanocluster

Tailoring the nanocluster at an atomic level leads to a tetrahedron-shaped FCC Pt1Ag28(S-Adm)18(PPh3)4 nanocluster and a large enhancement in photoluminescence.

Thus far, the atomic-level structural determination of bimetallic NCs by X-ray crystallography has only been achieved in a few cases. 11,20a,21b,22-26 The synthetic methods used to prepare bimetallic NCs can be roughly classied into two strategies: (1) the co-reduction of two metal precursors (e.g. complexes) in one-pot reactions 7,11,20,21b,22b,24 and (2) doping mono-metallic NCs (which serve as templates) with heteroatom complexes. 25a,29-32 For the synthesis of mono-metallic NCs, the thiol etching-induced transformation method is commonly used, 19,33 which gives rise to NCs with novel structures and distinct properties. However, this etching strategy has not been applied to the synthesis of alloy NCs. The etching strategy is highly attractive for alloy NCs because the heteroatom(s) can be regarded as labelling atom(s), which provide mechanistic insights into the etching process (similar to the isotope tracing method used in molecular chemistry).
Herein, we report the attainment of Pt 1 Ag 28 nanocluster coprotected by 1-adamantanethiolate (HS-Adm) and triphenyl phosphine (PPh 3 ) ligands, formulated as Pt 1 Ag 28 (S-Adm) 18 -(PPh 3 ) 4 . The Pt 1 Ag 28 nanoclusters are obtained by etching Pt 1 Ag 24 (SPhMe 2 ) 18 with both the HS-Adm and PPh 3 ligands, and the crystal structure of Pt 1 Ag 28 reveals that the Pt atom resides in the central position of the nanocluster. In addition, Pt 1 Ag 28 shows unique structural features including: (1) the presence of a face-centered cubic (FCC) Pt 1 Ag 12 kernel, which is observed for the rst time in silver-based alloy NCs, as opposed to the common icosahedral structure, and (2) the discovery of new surface motifs, such as the Ag 4 (SR) 6 (PPh 3 ) 1 motif and its assembled cage-like structure that protects the FCC kernel. Furthermore, compared to the Pt 1 Ag 24 precursor, the photoluminescence (PL) quantum yield (QY) of Pt 1 Ag 28 is largely increased from 0.1% to 4.9% (i.e. about 50 times of enhancement) due to the suppressed relaxation of the excited state via phonon emission and other non-radiative pathways. Moreover, a signicant enhancement in the thermal stability was also achieved in Pt 1 Ag 28 compared to that of the Pt 1 Ag 24 precursor nanocluster.

Synthesis of the [Pt 1 Ag 24 (SPhMe 2 ) 18 ](PPh 4 ) 2 nanocluster
For the nanocluster synthesis, AgNO 3 (30 mg, 0.18 mmol) was dissolved in 5 mL of CH 3 OH and 15 mL of CH 3 COOC 2 H 5 . H 2 PtCl 6 $6H 2 O (4 mg, 0.0075 mmol) was dissolved in 5 mL of CH 3 OH and added to the reaction mixture. The resulting solution was vigorously stirred (about 1200 rpm) with a magnetic stirrer bar for 15 min. Then, 100 mL of HSPhMe 2 was added. Aer another 15 min, 1 mL of NaBH 4 aqueous solution (20 mg mL À1 ) was added quickly to the reaction mixture under vigorous stirring. The reaction was allowed to proceed for 24 hours under a N 2 atmosphere. Aer the reaction was complete, the mixture in the organic phase was rotavaporated under vacuum, and then 20 mL of CH 3 OH was used to extract the product, which also contained the redundant HSPhMe 2 and by-products. 5 mL of a CH 3 OH solution containing excess PPh 4 Br was added into the abovementioned CH 3 OH solution. Subsequently, the resulting solution was centrifuged to obtain the solid. Approximately 15 mL of methanol was added to wash the synthesized nanocluster. The precipitate was then dissolved in CH 2 Cl 2 giving rise to [Pt 1 Ag 24 (SPhMe 2 ) 18 ](PPh 4 ) 2 nanoclusters (34 mg, 0.006 mmol, yield: 80.5% on a Ag mole basis).

(PPh 3 ) 4 nanocluster
For the nanocluster synthesis, 10 mg of (PPh 4 ) 2 -[Pt 1 Ag 24 (SPhMe 2 ) 18 ] was dissolved in 10 mL of CH 2 Cl 2 . Then, 5 mg of PPh 3 and 10 mg of AdmSH were added to the solution simultaneously. The reaction was allowed to proceed for 30 min at room temperature. The colour of solvent transformed from bright green to orange. The organic layer was separated from the precipitate and evaporated to dryness. The Pt 1 Ag 28 -(S-Adm) 18 (PPh 3 ) 4 nanocluster was obtained aerwards. The dried nanocluster was washed with methanol at least 3 times and collected by centrifugation (7 mg, 0.001 mmol, yield: 63.2% on a Ag mole basis).

Characterization
All UV-vis absorption spectra of the nanoclusters dissolved in CH 2 Cl 2 were recorded using an Agilent 8453 diode array spectrometer, whose background correction was made using a CH 2 Cl 2 blank. Solid samples were dissolved in CH 2 Cl 2 to make a dilute solution with its subsequent transformation to a 1 cm path length quartz cuvette, which was followed by the spectral measurements. Thermogravimetric analysis (TGA) was carried out on a thermogravimetric analyzer (DTG-60H, Shimadzu Instruments, Inc.) with 5 mg of nanocluster in a SiO 2 pan at a heating rate of 10 K min À1 from room temperature (about 298 K) to 1073 K. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250, congured with a monochromated Al Ka (1486.8 eV) 150 W X-ray source, 0.5 mm circular spot size, a ood gun to counter charging effects, and a analysis chamber base pressure lower than 1 Â 10 À9 mbar; data were collected at FAT ¼ 20 eV. Photoluminescence spectra were measured on a FL-4500 spectro-uorometer with the same optical density (OD) $0.05. In these experiments, the nanocluster solutions were prepared in CH 2 Cl 2 at a concentration of less than 1 mg mL À1 . Absolute quantum yields (QY) were measured using dilute solutions of the clusters (0.05 OD absorption at 480 nm) on a HORIBA Flu-oroMax-4P. Inductively coupled plasma-atomic emission spectrometry (ICP-AES) measurements were performed on an Atomscan Advantage instrument made by Thormo Jarrell Ash Corporation (USA). The nanoclusters were digested with concentrated nitric acid and the concentration of the nanoclusters was set to $0.5 mg L À1 .

Single-crystal growth and analysis
Single crystals of the Pt 1 Ag 28 (S-Adm) 18 (PPh 3 ) 4 nanocluster were grown at 4 C for 2-3 days in CH 2 Cl 2 /hexane. Red crystals were collected and the structures of Pt 1 Ag 28 (S-Adm) 18 (PPh 3 ) 4 were determined. The data collection for single crystal X-ray diffraction was carried out on a Bruker Smart APEX II CCD diffractometer under a liquid nitrogen ow at 150 K using graphitemonochromatized Cu Ka radiation (l ¼ 1.54178Å). Data reductions and absorption corrections were performed using the SAINT and SADABS programs, respectively. The structure was solved using direct methods and rened with full-matrix least squares on F 2 using the SHELXTL soware package. All non-hydrogen atoms were rened anisotropically, and all the hydrogen atoms were set in geometrically calculated positions and rened isotropically using the riding model.

Femto-nanosecond transient absorption spectra
Details of the femtosecond experiments have been described elsewhere. 37 Nanosecond transient absorption spectra were measured based on the same ultrafast pump pulses along with an electronically delayed supercontinuum light source with a sub-nanosecond pulse duration (EOS, Ultrafast Systems).

Time-correlated single-photon counting
Fluorescence lifetimes were measured using a time-correlated single photon counting (TCSPC) technique (Fluorolog-3 HORIBA Jobin Yvon); a pulsed LED source (376 nm, 1.1 ns) was used as the excitation source. The instrument response function (IRF) of detection was about 1.5 ns.

Results and discussion
Characterization of the reaction The reaction was monitored via UV-vis spectroscopy (Fig. 1A), in which the spectra show the gradual conversion of Pt 1 Ag 24 (SR) 18 to Pt 1 Ag 28 (S-Adm) 18 (PPh 3 ) 4 when reacting with Adm-SH and PPh 3 together as reected in the spectral changes. For a close comparison, the spectra of Pt 1 Ag 24 and the etching product Pt 1 Ag 28 are shown in Fig. 1B, in which one can see that the 465 and 600 nm absorption bands of Pt 1 Ag 24 are blue-shied to 445 and 545 nm, respectively, aer the conversion. In addition, we tested Adm-SH as the sole etching reagent in this reaction. The product was a mixture of larger nanoclusters as opposed to pure Pt 1 Ag 28 (Fig. S2 †).
Thermogravimetric analysis (TGA) of the product shows a total weight loss of 56.8 wt% (Fig. 1C), which is consistent with the theoretical loss (55.8 wt%) according to the formula determined via X-ray crystallography (vide infra); it should be noted that the loss of PPh 3 ligands accounts for 15.2 wt% and S-Adm ligands for 41.6 wt% (Fig. 1C), which is also consistent with the theoretical losses (14.4 and 41.4 wt%, respectively). The atomic ratio of Pt to Ag was analyzed via inductively coupled plasma (ICP) atomic emission spectroscopy to be Pt/Ag ¼ 3.9/96.1 and also by X-ray photoelectron spectroscopy (XPS) to be Pt/Ag ¼ 3.5/96.5, which are consistent with the expected ratio of Pt/Ag ¼ 3.5/96.5 (see Fig. 1D, S3 and Table S1 † for the data and details). Electrospray ionization mass spectrometry (ESI-MS) conrmed the purity of Pt 1 Ag 28 (Fig. S4 †), in the results of which only one peak was found (m/z ¼ 3637.67 Da, with z ¼ 2+ and perfectly matched the experimental and simulated isotope patterns of [Pt 1 Ag 28 (SR) 18 (PPh 3 ) 4 ] 2+ ).

Atomic structure
The structure of Pt 1 Ag 28 can be divided into two parts, the kernel and the surface shell. By comparing the crystal structures of the Pt 1 Ag 24 and Pt 1 Ag 28 nanoclusters, we identied that the six Ag 2 S 3 (-R groups omitted) staple motifs surrounding the Pt 1 Ag 12 kernel in the Pt 1 Ag 24 nanocluster change to the four Ag 4 S 6 P 1 motifs sharing six S atoms, forming an overall Ag 16 S 18 P 4 shell in a tetrahedral shape with the 4 motifs at the 4 vertices of tetrahedron (Fig. 2). As for the kernel structure, the icosahedral Pt 1 Ag 12 kernel of Pt 1 Ag 24 was converted into an FCC kernel (vide infra). The single Pt atom is surrounded by an Ag 12 shell to form the Pt 1 Ag 12 kernel. The Pt 1 Ag 12 kernel was further encircled by an integrated Ag 16 S 18 P 4 cage-like exterior shell. Thus, the entire structure shows a tri-stratied arrangement-Pt(center)@Ag 12 (shell)@Ag 16 S 18 P 4 (exterior). The bond lengths and bond angles are given in the ESI (Table S2 †).
For a better view, the overall Ag 16 S 18 P 4 shell was dissected into four equivalent Ag 4 S 6 P 1 motifs sharing six S atoms. The six S atoms in each Ag 4 S 6 P 1 motif are divided into two forms ( Fig. 2A and B): (1) the three S atoms (in red, vertically linking to the Ag atoms) bond to the Ag atoms in the kernel-shell (3 Â 4 ¼ 12Ag atoms in the M 13 kernel), which can be regarded as the bridges between the kernel and motif outside. The total 12S atoms stabilize the M 13 kernel in the overall structure; (2) the other three S atoms (in orange, connecting the bottom Ag atoms) act as linkers to connect two nearby Ag 4 S 6 P 1 motifs to  form the integrated motif shell-Ag 16 S 18 P 4 . Interestingly, the P atom and three bottom S atoms in each Ag 4 S 6 P 1 motif constitute a tetrahedral structure. The overall Ag 16 S 18 P 4 shell consists of four Ag 4 S 6 P 1 tetrahedral motifs (Fig. 2C), and the integrated conguration is also approximately a tetrahedron. The overall metal framework of Pt 1 Ag 28 (Fig. 2D) also adopts a tetrahedral shape, which is constructed by six 4-atom-long edges (Ag atoms from the shell) and four faces of Ag 3 (Ag atoms from the kernel). In addition, the angle between the edges is approximately 60 (Fig. 2D), which is consistent with the standard tetrahedral structure.
As to the structure of the Pt 1 Ag 12 kernel, the icosahedral kernel of Pt 1 Ag 24 was transformed to the FCC arrangement in Pt 1 Ag 28 aer the etching process. To the best of our knowledge, all the previously reported M 13 (M ¼ Au/Ag/Pt) kernels are icosahedral, and thus the FCC arrangement in the M 13 kernel was observed for the rst time in an alloy NC. Based on the well-dened structure of Pt 1 Ag 24 and Pt 1 Ag 28 , we propose a plausible mechanism for the transformation process. As shown in Fig. 3, the Pt 1 Ag 12 kernel of Pt 1 Ag 24 is in an icosahedral arrangement by the triangular shape of each face. When etched with Adm-SH and PPh 3 ligands, the relative positions of the Ag atoms on the kernel's surface shi, and consequently the bonds of the Pt 1 Ag 12 kernel in Pt 1 Ag 24 (i.e. bonds i-iii in Fig. 3A, 2.960-2.991Å) stretch to $3.610Å, indicating that the Ag-Ag bonds were broken. Simultaneously, the angle a enlarges from 73.6 to 84.7 . Thus, some of the triangular faces in the kernel of Pt 1 Ag 24 are rearranged into a quadrilateral in Pt 1 Ag 28 and the kernel is thus converted from an icosahedral to FCC arrangement. Furthermore, the slight distortion of the Pt 1 Ag 12 kernel in Pt 1 Ag 28 compared to the typical FCC M 13 kernel (Fig. 3C) was caused by the interaction between the kernel and the outside motifs.
As mentioned earlier, the heteroatom(s) can be used as labeling atom(s) to shed light on the mechanism of the structural transformation. In this study, the central position of the Pt atom (as the only heteroatom) was retained during the etching process. This phenomenon indicates that the M 13 kernel of Pt 1 Ag 24 does not fall apart in the etching process and instead it just becomes distorted in response to the transformation of the exterior motifs. On the other hand, the stability of the central Pt atom was also established in the Pt 1 Ag 24 and Pt 1 Au 24 cases using density functional theory (DFT) calculations and experimental studies. 20b, 28 Recently, an Ag 29 NC co-protected with BDT (1,3-benzenedithiol) and PPh 3 ligands as well as a single Au doped Au 1 Ag 28 NC have been reported by Bakr and co-workers. 22 In the present study, we discovered that the framework of Pt 1 Ag 28 was largely different from Ag 29 and Au 1 Ag 28 , albeit all of them have the same metal atom number (i.e. 29). The distinct differences in Pt 1 Ag 28 compared to the other two examples are manifested in the following (see Fig. S5-8 †): (1) the kernels of Ag 29 and its Audoped alloy are an icosahedral M 13 , whereas Pt 1 Ag 28 possesses an FCC Pt 1 Ag 12 kernel; (2) the motifs on the nanocluster surface are also entirely different; the Ag 29 (or Au 1 Ag 28 ) NC possesses four Ag 1 S 3 P 1 and four Ag 3 S 3 motifs (carbon tails omitted), whereas the Pt 1 Ag 28 is comprised of four new Ag 4 S 6 P 1 motifs. By sharing six thiolates, the four Ag 4 S 6 P 1 motifs form a cage-like Ag 16 S 18 P 4 structure; (3) in the Pt 1 Ag 28 nanocluster, all the metal atoms are located within the tetrahedron constructed via the four P atoms, while in Ag 29 and Au 1 Ag 28 , 12Ag atoms out of the 29 metal atoms overow the corresponding tetrahedron (i.e., only 17 atoms are completely contained in the tetrahedron). In addition, it should be noted that tetrahedron-shaped Au NCs have been studied previously 34 and our study lls in the blank in tetrahedral Ag NCs; (4) several smaller tetrahedral units were found in Pt 1 Ag 28 , such as the Ag 4 S 6 P 1 motifs, the assembled motif structure and the overall metal nanocluster structure; however, such tetrahedrons are not observed in the Ag 29 and Au 1 Ag 28 NCs; (5) a charge state of À3 was reported for Ag 29 and Au 1 Ag 28 with an electron count of 8e (that is, 29 À 24 + 3 ¼ 8e). In Pt 1 Ag 28 , ESI-MS (Fig. S4 †) identied that the cluster bears 2+ charges (not 3À in Ag 29 and Au 1 Ag 28 ), but the X-ray crystallographic analysis did not nd any counter ion (presumably Cl À disordered in the crystal). Taking the results together, the nominal electron count of Pt 1 Ag 28 is 8e (that is, 28 À 18 À 2 ¼ 8e).
The optical energy gaps and photoluminescence properties. To further compare the properties of the Pt 1 Ag 28 with Pt 1 Ag 24 , the absorption spectra and PL of both samples were analyzed. The energy-scale absorption spectra of Pt 1 Ag 24 and Pt 1 Ag 28 are shown in Fig. 4A, with the optical energy gap of Pt 1 Ag 24 being 1.72 eV and Pt 1 Ag 28 being 1.86 eV. To the naked eye, the solution color of Pt 1 Ag 24 is green, whereas the solution color of Pt 1 Ag 28 is orange (insets of Fig. 4A). The trend of the optical gap energies was a surprise since typically one would expect the larger size of Pt 1 Ag 28 to have a smaller gap than that of Pt 1 Ag 24 .
With respect to the PL properties (Fig. 4B), the QY of Pt 1 Ag 24 is very low (0.1%), 20b but interestingly the QY is largely increased to 4.9% in Pt 1 Ag 28 (by about a 50-fold enhancement). The PL of Pt 1 Ag 28 is strong enough to be perceived by the naked eye. In addition, the PL peak of Pt 1 Ag 24 was centered at 728 nm but it blue-shis to 672 nm in Pt 1 Ag 28 (a shi of ca. 56 nm). The PL excitation spectrum of Pt 1 Ag 28 was also measured, which is almost identical to its absorption spectrum (Fig. S9 †), indicating typical quantum connement behavior and electron relaxation to the LUMO level before uorescing.
The excited state behavior of Pt 1 Ag 24 and Pt 1 Ag 28 . Femtonanosecond transient absorption spectroscopy (fs-ns TA) and time-correlated single-photon counting (TCSPC) were performed on both the Pt 1 Ag 24 and Pt 1 Ag 28 nanoclusters in order to probe their excited state properties. Upon excitation at 360 nm, Pt 1 Ag 24 exhibits net ground state bleaching (GSB) at 450 nm and 580 nm, which corresponds to the UV-vis absorption and excited state absorption (ESA) around 500 nm and 700 nm (Fig. 5A). For Pt 1 Ag 28 (Fig. 5B), the TA spectra exhibit an ESA centered at 650 nm, a net GSB around 450 nm and a dip around 550 nm, which also agrees with the steady state absorption. The kinetic traces of the GSB around 450 nm for both nanoclusters were tted and compared (Fig. 5C).
For Pt 1 Ag 24 , the kinetic traces can be well tted by a single exponential decay with a time constant of 1.9 ms, whereas for Pt 1 Ag 28 , two exponential decays (300 ns, 2.9 ms) were required to obtain the best tting quality. Further femtosecond transient absorption measurements were performed, and the ultrafast relaxation dynamics for both clusters indeed exhibited similar behavior, which contain an ultrafast and a long lived decay (Fig. S10 †). The uorescence decays measured using timecorrelated single-photon counting (TCSPC) exhibit lifetimes similar to that obtained from ns-TA measurements (cf. Fig. 5C and D), which suggests that the decay components in Pt 1 Ag 24 and Pt 1 Ag 28 clusters are all radiative.
The long-lived decay in silver nanoclusters has been ascribed to ligand to metal charge transfer (LMCT) but the origin of which is not fully understood. 21b,22b, [35][36][37] As the crystal structures of Pt 1 Ag 24 ). The more drastic change in lifetime from Ag 29 to Pt 1 Ag 28 suggests that the electronic structure is more strongly modied in the case of Pt 1 Ag 28 , which may enhance the LMCT and lead to a higher uorescence quantum yield. Moreover, prominent coherent oscillations were observed in the femtosecond kinetic traces of Pt 1 Ag 24 , whereas no such phenomenon was observed for Pt 1 Ag 28 (Fig. S11 †). The stronger phonon emission observed in Pt 1 Ag 24 suggests that more excited-state energy is dissipated into the environment through heat, which also explains its weaker luminescence than that of Pt 1 Ag 28 .

Thermal stability
In addition to the PL properties, we further investigated the stability of Pt 1 Ag 24 and its etching product, Pt 1 Ag 28 (Fig. 6). The stability of these NCs was tested at 50 C in air (NCs dissolved in CHCl 3 ). As to Pt 1 Ag 28 , the UV-vis spectra were essentially unchanged over time (12 hours tested), which indicates its high stability, whereas the UV-vis spectra of Pt 1 Ag 24 signicantly decrease in intensity aer two hours and completely disappear in approximately six hours. The higher thermal stability of Pt 1 Ag 28 than that of Pt 1 Ag 24 was ascribed to the more robust tetrahedral structure than the icosahedral one of Pt 1 Ag 24 .
The stability of Pt 1 Ag 24 and Pt 1 Ag 28 was also characterized via TGA measurements. As depicted in Fig. S12b and d, † the maximum weight loss temperature of Pt 1 Ag 24 was 240 C (i.e. the derivative curve), which was much lower than that of Pt 1 Ag 28 (310 C). It should be noted that the 220 C peak in Pt 1 Ag 28  corresponds to the loss of PPh 3 ligands, which are easier to lose compared to the thiolate ligands. Bakr and co-workers reported that the lack of PPh 3 ligands did not alter the overall conguration of Ag 29 and Au 1 Au 28 . 22 Thus, we suspect that the conguration of Pt 1 Ag 28 was maintained at this stage. These results indicate higher stability of Pt 1 Ag 28 compared to that of Pt 1 Ag 24 .

Conclusion
In summary, we devised an etching method for the conversion of Pt 1 Ag 24 (SPhMe 2 ) 18 to Pt 1 Ag 28 (S-Adm) 18 (PPh 3 ) 4 in the presence of Adm-SH and PPh 3 . The central Pt atom is retained in the conversion process; however, the Pt 1 Ag 12 kernel was converted from an icosahedron to FCC arrangement, which is observed for the rst time in the M 13 kernel of alloy NCs. Multiple tetrahedral motifs were identied in the alloy NC, such as the Ag 4 S 6 P 1 surface motif, the integrated motif shell Ag 16 S 18 P 4 , and the overall metal framework. The PL QY is signicantly increased from only 0.1% for Pt 1 Ag 24 to 4.9% for Pt 1 Ag 28 (about a 50-fold enhancement). The ultrafast dynamics results reveal that the enhanced luminescence of Pt 1 Ag 28 was due to the suppressed phonon emission and other non-radiative pathways in the tetrahedral structure. In addition, the thermal stability of Pt 1 Ag 28 was drastically enhanced compared to that of its precursor, Pt 1 Ag 24 . It is hoped that this study will help stimulate the future discovery of new alloy NCs with tailored functionalities for wide applications in sensing and energy elds.